When I replace U-joints, I always seat them by driving the cups outward so the clip fit tight against the driveshaft ears. To do that, I use a vice as an anvil. Open the jaws until it's the right width to catch both ears on one plane and smack the driveshaft with a rubber mallet right on the end of the tube where it's welded to it's half of the yoke. Flip it over and do the same to seat the other cup. Rotate 90 degrees and do the other plane the same way. That should free them up. If not, it may be that the ears are bent in a bit, or, there may be a burr or some slag keeping the clip from seating against it's ear. This applies to the standard and common internal clip type U-joints, but, a similar procedure will work on the external clip type joints too.
