Take the bowl off and check the top of the bowl and the rim where it seats, you may have a small bit of muck that is stopping it seal, or may have a small nick out of it, i have seen gaskets for this not sure if there a after market thing, never had to use one. be careful you don't try to solve the leak by over doing the screws that hold it on, you break a screw off and you will be in trouble.
Then ptfe around the brass screw as said before.