With the carrier in your hand, and no axle in it, the spacer tube should be snug in between the bearings. If it's loose, when you tighten down the nuts, it puts the bearings in a bind. I have seen a carrier before that had a previous catastrophic bearing failure and the bearing shoulder that the bearing is supposed to press against was mooshed over which made the distance between the shoulders wider than the length of the tube. If the bearings won't press in far enough to make the tube snug, you need to take the bearings out and inspect. It's also possible for the tube to be worn down on the ends from rattling around.