Is it possible that Fatty is laying the eggs and the other one is eating them and then spitting them up? FBTs will eat their eggs, tadpoles, etc. Also, what is your substrate? Is it small enough for them to accidently swallow? If so, that could be the problem.
I've got a similar problem with 2 of my females in that they gained weight but no eggs yet. I have an email in to a professional right now about the problem. If I hear back from him, I'll post it for you to read.
Good luck!

