Try and explain it best I can..
The cob does easily pop out. The process I use is a extra step compared to putting the TIM on while in the holder.
They all WILL stay in. Some more so then others
1. Put TIM on COB as evenly and thinly as possible out of holder
2. Pick up COB by edges trying not to get too messy
3. place correct edge on side with spring lever
4. Lay the COB flat on holder while keeping the spring lever engaged
5. Snap into place. Sometimes you need to move it around a bit to get the metal clips to dig into the cob sides
6. Go back and touch up the TIM which will have probably got messed up a bit. Its easier for me to do a touch up then apply all the TIM in the holder without it popping out.
7. Gently flip over and place exactly where it needs to sit. You can't really move it much without it popping out or messing up the TIM. You also have to screw in a little each side at a time or torquing one end will make it pop out
Thats about how I go about it..
Be sure to wear some thin medical latex gloves or something...the TIM is toxic..
Have installed quite a few holders and I've never had the contacts not meet up when properly seated. Maybe bend them if need be..
Also check out Pacific Light Concepts in the ads, they have the bjb holders which are probably the best out.