I purchased a Nexstar Select Trainer and flew it a few times and then put it up for a year or so. Recently I get it back out and flew a couple of times with no problems and then took the thing apart to clean it and never run right since.
Nobody has addressed this point yet. If you only flew a few times, then put it away, and tried to fly a few more times, there is a distinct possibility your fuel has gone bad. I've had this happen to me. Try some new fuel.
You make no mention of your efforts to tune the engine. More than one RTF trainer has been sent out with insufficient travel on the limiters to be able to correctly tune the engine. I believe the Nexstar select falls into this category as well. Get rid of the limiters and get someone with some experience to help you tune the engine.
Brad