quarter turn monobloc kitchen taps question

About a year ago I fitted a new monobloc kitchen tap - the quarter turn jobbies. I have found that whilst the hot tap turns on and off fine, the cold tap is getting very tight to turn either on or off. I took out the valve and had a squint, and all seems ok, when not in it's housing the value turns on and off easily, but as soon as its returned to the body of the tap its hard to turn - any ideas ??

Is the cartridge brass or plastic?

Thanks for the swift reply watertight - I cant recall but 80% sure its plastic - what difference does it make, and what do you think causes it ? - Cheers