The bearings are a very snug slip fit and it's important to keep them very straight and even as they go on. If even slightly cocked, they can bind and not move. Do not force them, try wiggling them by hand but if it cocks and stops, pull it back off and try again. a little smear of light oil can help, but mostly try to keep them straight. This is, of course, assuming they are the right size.
Dave