Originally Posted by FBinNY

Steerers are usually pretty well toleranced very close to the nominal so I doubt it's the steerer. I can't make a judgement call without seeing the fit, but maybe it's a matter of carefully shaving the forward edge so it'll start, but unlike split aluminum cones which constrict as they slide in, most plastic rings are made to actually compress (crush) to establish the tight fit and centering of the upper bearing.