1. take your left side selector off
2. take a tiny drill bit and, with just finger pressure/twisting, clean up and slightly deepen the detent holes in the body...they're probably worn (plastic bodys tend to do that).
3. take a look at the nub in the selector to see that it's still there...and not worn. If it's a ball bearing, then you're fine...if it's a plastic nub, you can reprofile it a little bit but it's a huge pain in the ass.
4. reassemble and test....most times simply cleaning up the detent holes will make it clickity-click again.
If the selector needs to be pressed tighter against the body, see if you can reassemble that way. If not you may need to shave the end of the selector rod just a little.