the plastic guide rod may not be working up and down freely. hard water can cause a build up. my toilet was getting harder and harder to flush because it was sticking. the force needed to lift the canister up was increasing because of the sticking. I replaced the gasket on the bottom of the canister and while i had the canister out of the toilet, i noticed the guide rod was sticking. after closer inspection it had a buildup of calcium. i cleaned it and put a little bit of plumbers grease on the rod. there are videos on youtube showing how to remove the canister. after i reinstalled the canister, it flushed like it was brand new, silky smooth