Perhaps you are in manual mode, and haven't compensated for the loss of light - so the pictures will be under-exposed...? If you can, please post a sample image here, it sure would help in finding out what your problem is.

#1. Also, what kind of lighting were you photographing in? I have used a circular polarizer for indoor portraits, and the results weren't very pleasant.

#2. Check to see what the "White Balance" in your camera is set to. The wrong WB can produce photos with strange colors.

#3. Spinning the filter varies the intensity of its polarizing effect.