I originally had it set up with a 50/47, the rings are steel, the rear is 16/18/20/22 and there was no problem with shifting between the rings, up or down on any of the rear gears.

I have some hilly rides planned and I want to switch to some alpine gearing. The new rings are aluminum, 48/45 and the rear is 17/20/24/28. It shifts fine from big to little but won't engage little to big.

I have gone up and down with the FD on the seat tube with no luck. There is a difference with the ring spacing. The aluminum rings are spaced about 6 mm c-c and the steel rings are about 5 mm c-c. My chain may be too short but there seems be enough play in the RD, I could be wrong though. Any ideas?

Here is a picture of the FD, if that is of any help.