Hello:
This isn't necessarily an issue with charge balance. It could be that you've defined a system that is far from equilibrium, and thus has trouble converging. If the pH value is important, I recommend starting with 3.5, so the system initially converges, then sliding the pH (in the Reactants pane) down to 3.1, so that you have a stable chemistry.
Hope that helps,
Tom Meuzelaar
RockWare, Inc.