Bicycle Mechanics - Broken Ultegra 600 STI ?

I've just bought a second hand Ultegra STI and the right shifter only works one way - I can use the large lever to change sprockets but pressure on the smaller lever doesn't do anything and I can't change down. It moves freely - but there is no pressure against the lever - I guess it isn't touching/meshing with what it needs to.
This kit is probably a few years old - but not sure as I can't find out any details (they're the style with the round shimano badge on the front, not like the new angled shiny things!)
I've taken th things apart and can't see anything wrong - but it appears that the internal ratchet isn't releasing the cable tension- but can't see what moves those ratchets!
Has anyone got any ideas - are they just worn out?

Shimano class these as 'not user serviceable'. Technically, it's probably possible to replace the ratchet, but I think unless you've got a couple of decade's worth of watchmaking skills, you'll find an STI repair a bit beyond the resources of the average mechanic.

I have two such R shifters, an 8spd that stops at the 6th detent and a 9spd that stops at the 8th. As Neil says Shimano STI repair is a formidable task and no
spare parts in the area of failure are available so you would have to cannibalize a good one. There are a very few user/LBS pro mechanic fixable things to an STI but
ratchet problems are not one. Campy is in theory more
repairable. My experience is life span of R STI shifter is 8-12kmi but that is like chain life. Shimano changes something about the shifters every 2-3yrs anyway. The 8spd shifters are only available in bottom end groups. Now with 10spd coming out wonder about where the 9spd shifters will end up in 5yrs. Steve

Someone's advice that a good spray of WD40 did the trick!
I had got too much grease on one of those tiny little ratchets - the spring is so weak that a big blob of grease was stopping it from dropping into the ratchet. A few sprays removed the excess grease and hey presto - STI that now changes both up and down. Cool!
I must admit that putting them back together was a pain too - fitting that spring for the main lever tension took ages!

I've revived several with just a spritz of WC40 and a blast of compressed air to blow out the cobwebs. If it doesn't work anyway, what have you got to lose?