I ran into this problem several months ago. I just shook it and forced it until it retracted and never thought anything of it. The trimmer recently broke (still functioned but extended beyond its normal point) and I took it apart. I realized that there were a few broken parts inside (possibly because of my earlier experience). So I removed the few parts and now it still extends to far, but otherwise functions ok if I'm careful.
I would recommend taking yours apart and seeing if you can find any parts floating around which may be causing it to not retract all the way. It may be broken beyond complete repair, but you may be able to get a little more use out of it.
If nothing else, you can order a new casing for it from here
Here are the instructions for taking it apart:
The shaver itself is easy to take apart. The part on the bottom(opposite from the shaver mechanism) pops off with just a slight bit ofprying. Be careful not to break the pins though. Once you have thatoff, the narrow sides pop off pretty easily. Then you can remove the 4screws opening up the interior.
Be careful at this pointbecause there are a lot of parts that may fall out. (Perhaps take apicture as soon as you open it so you know where everything was whenyou opened it). At this point you can try your hand at fixing it (minehad 3 parts broken and loose inside), or you can simply switch it withthe new casing (Which i'm going to do when I get it). Good Luck!