Anyone knows what's the most usual reason that the keys of the Yamaha P-200 brake?

I bought my Yamaha P-200 in January 2003. Since then, I have had it repaired 4 times (!!!!). Everytime the problem has been that the keys get's stucked.
I suspect that the problems have occured because of hits against the "back" of the piano. Is this the usual way, or it hits directly on the "keys" the mainreason?

I use a soft case (Warwick).

My question is if you are an owner of a P-200, and have you occured the same problem? Any suggestions?