To consistently generate variations from a part, it is important that the original be free of geometric inconsistencies that could be carried over into the variations. You should always look out for inconsistent geometry in any part that you intend to use as a master part. If you find inconsistencies, either remove them by using Clean Pnt, Clean Duplicate, and Clean Stacked in the Modify group on the Parametric tab, or isolate problem areas from consideration by the solver by collecting these entities into rigid bodies.