I had a similar problem to something raised elswhere in this forum with a delta tub faucet set, but that persons solution definitely doesn't work for me. I NEED HELP! The faucet has a plastic stem with a stainless steel base that rotates over a rubber seat and a stainless steel spring to turn the faucet on and off. Hot and cold are the same mechanism and are separately. The stem is tightened in place by a brass nut. I had a small leak so changed out the seat and spring. Ever since I can't get the system to turn off, and I AM POSITIVE I HAVE IT INSTALLED RIGHT! There is a tiny difference in the height of the seat relative to the original, and I suspect this is the problem--that I'm getting the wrong part. Both places I've been to assure me this is the right part (admittedly, home despot and Lowe's so not exactly authorities). Are there subtle differences in these parts? If so, shame on delta for producing problems like this. Can anyone help me here?

Hot and cold may not be the same on a Delta. Make sure you have the right cartridge installed. If it is installed in the wrong side, the handle and stop are 180* off.