Try wiggling it, and rotating it back and forth. I was having similar trouble as well when installing it today. I was tempted by the rubber mallet but managed to get it right with a bit of wiggle.
I think the act of tightening and untightening the barrel nut deforms the aluminum nut a little. Given how close the fit is between the nut and the tube, that may be why it becomes more difficult. In my case, I had trouble with my torque wrench and went over 40 ft-lbs. The nut deformed slightly at the spanner hole and hence the tube required some wiggle and elbow grease to get back on.
In the end though, it looks great, lines up perfectly, and is easier to install that a lot of other rails. I think the SMR has superceded the URX III as my favorite rail.