Make sure that your sidechain channel and your compressor channel are matched - i.e both either in stereo or mono.

If you sidechain a mono kick to a stereo synth the compressor will go wierd, choppy or might not work at all.

Also, be sure that you are not overloading your CPU. This is always the first plugin to complain if my CPU levels creep up. Trying turning off any inserts you might have, and mixing down the kick and the synth (assuming that it what you are trying to sidechain)

Best o' luck...