Could be any number of things. Here are a couple of guides that will aid you in detail D&R. The thumb safety should pop right out if you have it in the right position... if you need to use force, you're doing it wrong.
http://www.m1911.org/stripin1.htm
http://www.10-8performance.com/pages...27s-Guide.html
Pay close attention to the disconnector and sear installation as well as the sear spring.
Cheers,
C