Chances are the theaded end of the axle has mushroomed out a little, just enougjh to cause the nut to be too hard to turn. Try puting the nut on with some grease on the threads to make it eaiser. IF it feels like it is going on straight, just keep turinging and it will get passed the mushed part. BUt, if it feels real hard, it might stripp the threads. You can also try working the nut back and forth, just be carfull that it is on straight or you might not be able take it back off, or put it on further.
