Hi mechanics, I have a flat bar road bike with thumb shifters (shimano 200CX) they have one lever to shift up to the next largest cog and another lever to drop to a smaller cog. The rear lever won't engage, after some fiddling it does, then after a few shifts it won't engage again. I'm guessing this is a common failure and was wondering if it's fixable or do I just need to buy some new shifters?