Take it apart again. See if the washer is loose, of was accidentally damaged on assembly. Verify that the seat surface is perfectly smooth. Actually, take the seat out and reinstall with a wrap of teflon tape on the threads. Be sure to trim the excess tape. This is theoretically not required, but I find it helpful.
Be sure when reinstalling stems that they are turned to the open position. Otherwise, the washer bottoms out before the stem is tightly secured, thus damaging the washer and possibly the seat.