Pull the barrel and measure the following:

1. How much of the go-gauge sticks out of the chamber
2. How far your FL sized brass sticks out of the chamber

Could be you got a slightly short gauge, a slightly long set of dies, or the chamber in the barrel is a little short. The gauge and FL sized brass should both stick out roughly the same amount, precisely 0.120 to 0.125" IIRC.