Trouble mounting 1.4X TC II on new 500mm f/4

I just received my 500 mm f/4 and when trying to attach the Canon 1.4x TC II it goes on easily but doesn't lock. I've tried taking it off and reapplying without success. I give it a little nudge but don't want to force it obviously. Has anyone else had this problem and if so, any suggestions for resolution? I use the 1.4TC on my 300mm all the time with no problem and the 500 mm attachs and locks to the camera (30D), no problem. Thanks in advance for any assistance.
~Michele

I have had a similar experience with my 500/1.4tc setup - I now push the
latch button forward and make sure its seated before I attach the camera.
Its been an intermittent condition, so now I always check, just like checking
QR knobs and the like.

Thanks much, ELT. I'll just have to be extra careful

The locking pin is attached to the spring-loaded release lever on the tc and moves in and out together. If this mechanism is free to move and the pin is long enough, it HAS to drop into the locating hole machined into the 500 mount. If the tc is not locking either:

1. You are not turning the tc onto the 500 enough for the pin to drop into the locating hole. Just keep turning until you hear the pin drop into the locating hole with a little click.
2. The pin is not free to move in and out or the pin is not spring loaded so does not drop into the locating hole (move the release lever on the tc in and out a few times to free up- maybe it's dirty. Check that there is still spring in the system).

Thanks, yes, there is still spring. I tried ELTs recommendation to pull the release lever as I attach the TC and push it back when the TC is seated. It works that way. On my 300 mm I don't seem to have to be so careful, just pull the release button and it closes/seats/clicks on its own, not so with the 500. Maybe it just needs to get worn in a little?