Sometimes there's a bit of distortion, or the stem doesn't relax open completely even with the bolts removed. There could also be a small but or rolled edge at the stop of the steerer.

Odds are you can get it on with a bit of gentle persuasion. Or you may gently spread the stem clamp within it's spring range. One way to do this is to put a penny into the slot and put the screw in from the outside on the threaded side so tightening against the penny jacks the clamp apart. Don't get carried away, you want to do this only within the stems spring range range or a millimeter or so.