greasing uj's

i've just been trying to grease my props but i can't get any grease into two of the u j's. i've removed, cleaned and tested the nipples and grease goes through fine but when i screw them back in and try to grease the joint the grease just spews out around the nozzle of the grease gun with no sign of any entering the joint. i replaced the nozzle of my grease gun to see if a better fit would help but made no difference, no probs on front uj or prop sleeves just the two joints on the transfer box.
any suggestions greatly apreiciated.

Take the nipple off and then put it in.

I'm a qualified ex-RAF grease monkey. If the grease gun works fine on other nipples there's probably nothing wrong with the gun. try swapping the nipples from those that work. Sometimes the nipples don't make a perfect seal in the grease gun socket. the grease sometimes goes hard in the capillaries and joints or blocks with rust. If you cant force grease in then its usualy time to change

I had some trouble doing this too, exactly as you describe. I was using a flexible end on the grease gun which kind of locked onto the nipple. I figured it was coz the seal wasn't tight enough but I couldn't hold it on and pump (would need 3 hands) so I tried using a rigid end on the gun so I could push it tight onto the nipple and pumped. If the grease didn't go into the joint, I don't know where it went!