Yes, it might be the case but without a minimal version of your definition I can only speculate. It also might be that your model is close to the computational time limit and in some cases times out.
In most cases, our servers can’t perform an exhaustive checking of all possible parameter sets. Therefore it is possible that model checking succeeds even though some parameter sets take longer than the time limit to compute. In that case, the viewer will display an error message during the viewing session instead of displaying the result. More in the Documentation.