Hi

I've just returned to the world of biking after a long absence and am new to the world of index gears (it has been a LONG time since I rode)

Anyway, I have a problem with the rear derailer. Its a seven cog set and I can select every gear except the fifth (3rd smallest). If I change down from 6th to 5th its stays in 6th.

Can anyone give me some pointers on what I might need to adjust or a good guide that may help before I launch myself at it with a screwdriver?

Any help greatly appreciated