Recently, the last two times, when I have filled my gas tank the auto shut off doesn't shut off the nozzle from the pump and gas leaks out from under the Blazer. I have used two different pumps and the same thing happens. My guess is that the fuel is leaking around the filler neck but why would it prevent the automatic cut-off from working?

 

Don't guess when it comes fuel leaks, find out where it's leaking, and repair it. Also, check the vent line for a restriction.