Problem with trills

[jmao]

Hi, I asked before for some tips to improve my trills and they said I should use not only my fingers but my forearm as well.
I've been practicing, and yes they've improved a lot. But now I have a different problem.
I can play short trills but when I have to play a long trill I can't make it to the end. I play with my 2-3 fingers. What happens is that I reach I point where the note that's played with the 2nd finger doesn't sound but the note I play with the 3rd finger still sounds.

Do you have any tips to solve this problem?.  Right now I'm playing Mozart sonata k332, and I can't get to play the trills in the second movement correctly.

Thanks in advance