To eliminate that, put the check on the bench, push the base onto the check, and leave the boolit standing up on the bench.
You can see it its off of being seated flat. Ones that are- I push 'em down a little harder.
If they're still 'difficult', I lay that flat wrench for the sizer on top of the die, and push it down square with the press.
There's a little tool for doing that, but I never felt the need for it.
Also, you can't find a picture of it--- that probably means it would be real simple to make.