Check and see if fuel has infiltrated into your float. If it is heavy with gas it wont seat properly. Shake it next to your ear and listen or hold up against a bright light to see if there is gas in it. Also, as Jay said, your seat can get gummed up with shit. You can use a toothpick or blunt end of a q tip to kind of swab out the seating surface. Check and make sure the end of your needle is very clean as well. Sounds to me your float has a leak (gas getting in it) or your needle seat is gummed up with shit. Lets us know how ya make out.