Ah I see, I completely missed reading that it is a time dependent Neumann BC and not Dirichlet. What I wrote above is for Dirichlet BC. Nevermind !
As for your question, is the solve inside or outside the loop? Because with the code you have posted there should be nothing that requires any recompilation of the forms. Can you create a minimal working example instead? See Read before posting: How do I get my question answered?