Till you figure it out, you could try and tighten the screw every so often, like after every so many blocks, or every 15 minutes. Not ideal, but you can finish your top.
The other thing that can make the needle snap is if your thread gets caught somewhere on the machine, so if it breaks again, make sure that it was threaded properly.
I've never heard of a walking foot that doesn't have a fork or prong that goes over or around the needle screw. If you have a generic one, however, I would invest in the brand name one for your machine.