Monte Carlo Simulation - NormInv function

I know past versions of excel (almost exclusively pre-2007) have been criticized for problems with some of its statistical functions. (Random number generators that fail randomization tests and inefficient and unstable algorithms in the standard deviation and regression algorithms for example.) But I've not heard of problems with Excel's normal distribution functions -- especially in the newest versions. Which isn't to say it is impossible that Excel's implementation of the normal distribution isn't problematic, just that I'm unaware of any problems.

I'm not sure I can contribute much else. You haven't shared any details of your model (perhaps someone with expertise in your field needs no more detai?), So I'm not sure how one would go about debugging this so that one could determine a problem with Excel's normal distribution functions. We might have a couple with the necessary expertise. The rest of us would require more details in order to understand the problem.