I'm not familiar with the specific problem you're experiencing. But have some questions for you to think about (not necessarily answer me, I'm not going to trouble-shoot your bike for you) and some advice.

1) How often/how many times have you changed the cabling? When I hear about shifters/derailleurs "locking" I immediately think about the cabling.
2) Have you ever taken the shifter off and sprayed the inside with compressed air? It's possible for sand grains to get into the internals and mess with them. Maybe spray some WD-40 in there followed by a good burst of compressed air would work that out.
3) Obviously make sure the wheel is seated correctly - cassette at an angle would cause issues with shifting.
4) You're going to be much better off (in my opinion) finding two matching new/used shifters. This will make your life easy and you really won't spend too much more money. I would go for the used ones. Lots of people buy a new bike and immediate switch out the shifters/derailleurs for somethign else. i.e. "Like new". Can get this stuff reasonably cheap.