First, you'd probably get better answers if you asked this in the actual Simulate section of the Forums. Those guys make a living doing this stuff.
Second, without an idea what your model is, it's pretty hard to have a clue as to what might be the trouble. Maybe the surfaces are non-manifold? Maybe there are small features like fillets that are not meshable? Maybe the surfaces are self-intersecting?
Third, just knowing it "can't mesh it" is not enough detail to understand anything. It's the generic error message for just about any model that is failing.