Is the geometry complete and does it extend far enough upstream and downstream, so as to allow the flow to develop properly, e.g. non-convergence is common in cases where the outlet zone is too short and a recirculation zone exists near the outlet. Similarly, at an inlet, consider modelling the flow sufficiently far upstream so that a fully developed flow profile approximation is sensible.
There may also be unimportant geometrical features which could make the results difficult to converge. Consider simplifying the geometry and removing these features.