Hi Michael,
I am getting the idea that you are changing the accuracy of the assembly. If so then maybe look at the accuracies of the large parts first. The setting depends (I think) on whether you are working in furlongs per fortnight or metric (guess which one I use). I think Frank's suggestion should be OK for Imperial. Once all parts are at the same accuracy then the assembly should work.
Regards, Brent
Oh yes. You say Creo but what version and cut just in case that is relevant.