Anyone re-lubed their APR RSB yet? Having problems with the zerk fitting.

[FliGi7]

Since the APR MKV RSB comes with zerk fittings, I wanted to take the opportunity to re-lube it since it's been squeaking for a while now. I even took the time and effort to buy the Polyurethane-specific silicon grease tube online for use in a standard zerk gun. The cartridge arrived and I picked up a standard size zerk gun from the store with the flex tube to get in there. I put it all together and got under the car and the zerk fitting won't attach. It simply will not latch on to the nipple. I'm not really sure what's going on and why it won't latch but I've been under there several times now and it won't click and snap on so the grease will go into the fitting.

Any experiences like this, or successful ones?