What part numbers did you buy? When selecting bearings and shafts, you always have to check the dimensional tolerances of mating features.
Doing a quick check of the 1600 inch series bearing datasheet
here shows that the bore size range is .3125/.3120 . Drill rod from Mcmaster Carr is .3130/.3120 . A tolerance stackup calculation shows your fit could range from a .0005 slip fit to a .0010 press fit.
I have to point out that tolerance stackups are the most basic calculations a mechanical designer needs to do. I will also admit that I didn't check a single bearing fit for any of the stuff we ordered, or teach my students how to do them. (smack forehead) One more way I need to be a better mentor...