Wont shut off
I installed 6 of these at one house on old valve bodies. One of the six wouldn't shut off all the way. I could hear the water seeping through it. Here's what worked for me. Turning the whole unit clockwise is similar to using a flow control. You can actually shut off the water flow completley. I had to turn it way down before the seeping would stop. If I'd turned it anymore the valve wouldn't have functioned. I wasn't sure at all this would be a long term fix. It's been many months now and it's still working. -- You also might have debris between the seat washer and the seat. Maybe you should take the unit off and flush the line. Look at the seat washer for pits and the seat to see if it's smooth. You didn't per chance install it on a 1" valve body with a 3/4" seat washer did you?. You do have this installed on a "brass" valve body don't you? If all else fails I'd take it back and get a new one.
Good luck!
If I can't fix it, it's broken!