Early 80's Bianchi with a Campy Super Record rear derailleur. It doesn't want to shift down to the smallest cog when I'm on the large chain ring. When I'm on the small front chain ring it shifts down perfectly fine.
The only thing I noticed is when on the large ring the upper jockey wheel is farther away from the freewheel due to chain tension so that the upper jockey wheel can't convince the chain to make the jump down one more cog. The chain and freewheel are both pretty new.
My only thought is a few more links added to the chain might help the situation by not stretching out the jockey wheel cage so much.

Post a picture of the gear train , in high and low range.

Is the OP aware of how the hi-limit screw functions, and of cable tension adjustment?

These are the usual approaches to lazy shifting toward the small end of the freewheel. The chain gap is expected to vary with chainring choice, but the rear derailer should be able to shift using either ring, especially the large ring.

Changes in chain length or axle position should take into consideration the chain's tension at all combinations of sprockets.

If the guide pulley is far away from the largest cog try loosening the "B" adjustment screw in the back. It changes the angle of the derailleur. That doesn't sound like it's causing you to not be able to shift to smallest cog, but you never know. Could be a contributing factor.

If it's the Campy SuperRecord RD I'm thinking of, then there is no B adj screw. That all came later. I've decided that any time an RD is not working well is the time to ensure the alignments are correct. The dropouts need to be parallel and the RD hanger needs to be right. And it's worth being fussy about those alignments - measured carefully vs just eyeball'n it. Then be sure the chain length is correct - add or delete one link pair and move the axle in the slot to optimize the length at the 'four corners' of the transmission (big-big, big-small, small-small, small-big). Rather than keep pushing pins in and out, I remove my Missing Link and use a hooked wire to close the chain and move the hooks to 'add and delete' links. When I have the length I then push pins.

I've even gone so far as to grab two lengths of thin wood (sorta like chop sticks, I keep that sort of thing around) and clip one length to the large cog and the other to the pulley cage. The wood strips are straight and about 8 inches long. They serve as 'winding sticks' that really highlight any mis-alignment between the RD and the FW.

Yes, I'm really boring to talk to at cocktail parties and family reunions.

[MENTION=360474]Prowler[/MENTION] - highly doubtful that you're boring . I neglected to connect to the fact that it is a Campy derailleur. I know nothing about them and may have added to the OP's confusion. Oops.

Glad you posted correct specifics.

"cable tension " is not relevant in this case, since it's vintage non-indexing.

OP, you could try letting out the High limit screw a bit more, even though it will be a tiny bit too much when on the small ring, but riding small-small is not that common, and even still, it may be fine.

This is what I've done and everything works fine. I guess I just expected that the limit screw settings that worked on one chainring would be just fine for the other chainring as well. I see now there is some compromise that needs to be expected.
I was unable to add a few chain links. 1 link would have been fine, 2 links were too long.

This is what friction shifters and classic and vintage is all about though, trimming the shifters for each gear selection!

Well, you do have to have an even number of links.

It is curious that the shifting was worse on the big ring. With a Rally-style (or Shimano Crane GS) this to be expected. They work by moving the guide pulley up and down intentionally, but the big ring moves the pulley downward without changing the cog. With your RD that shouldn't be the case. In fact, poor shifting suggests that the guide pulley has moved too far away from the cog, which suggests your chain is too short. Normally I'd expect a short cage NR style RD to move the pulley closer to the cogs when you go to the big ring.

Its vey relevant. If the cable is too tight the RD wont rest firmly against the stop.

The OP's issues are classic symptoms of worn out jockey wheels, a tight cable or too short of a chain.

The chain is the longest it can be and the cable allows the RD to rest on the stop. I'll look into the jockey wheel situation. They aren't sloppy on their bearings and have minimal side to side play but maybe the cog profile is too worn down. I don't have any new ones to compare them to.

Even if the RD is resting on the stop the cable can still be too tight. A too tight cable will cause the RD to lazily come to a stop. When you test the upper pulley rock it...leading edge in, trailing edge out then leading edge out, trailing edge in.

^+1
I've set up RDs with no slack at all in the small cog position and had the same problem. Backed off the RD cable a bit so there's no tension in that last position and the problem went away.

Yes, worth looking at, if that is not it...
Take a gloved hand (I really don't like getting my hands filthy on purpose) and twist the chain to see how much it flexes in rotation axialy. Always a chance the chain is toast. I recently had that problem on a bike I bought, was not stretched, but was way too limber the way I described. New chain changed all.

"Cable tension" is usually used to refer to micro adjusting the derailleur on indexed systems, but most retro bikes don't have an adjusting cable stop.
I do agree though, that whatever term, make sure the derailleur can freely hit the 'H' stop.
Campy derailleurs did sometimes have trouble hitting the smallest cog reliably.

I'll give that a try. Mine has little slack at all when resting on the stop.

Actually, many retro RDs did, perhaps most. Depending on the setup you may be able to install one even if the RD didn't. But the problem can be real; we try to have cables which don't go too slack with the lever all the way forward but sometimes we leave them just a bit tight. But that doesn't necessarily explain why the big ring is problematic and the small ring isn't.

+1 on the benefit of a new chain.