Our bikes have been in storage for a year or so. Today I got them out to start reconditioning, and found that the front shifter on BOTH bikes has the same problem. Both bikes are essentially mountain bikes, straight handlebar, thumb shifters. On both, when you push the large thumb lever on the left to shift to the next higher front ring, the cage moves the chain up onto the next ring, but it will not stay there. When you remove your thumb, the cage goes back down to its inner most position over the smallest ring. If I remember right, there is supposed to be some sort of ratcheting action that keeps the cage over successively larger front rings as you shift up to higher gears. Can you shed any light on this?
Thanks for the email, John. You're right, there's supposed to be something that holds the derailleur in gear and that something is the shift lever. The derailleurs have springs in them and the only thing that keeps the derailleur in gear is the shift lever. If the lever loosens up, the spring will overpower it and the derailleur will shift right back out of gear the second you let go of the lever.
You didn't tell me what type of shift lever you have, but most basic thumb shifters have either a regular screw on top of them, or a screw with a D-ring attached to it that you lift up to turn. To tighten the lever and keep tension on it to hold the derailleur in gear you turn the screw with a screwdriver (or the screw with the D-ring) clockwise. It should be that simple.
Hope this solves the problem,