A couple things.

Crank brothers have adjustable float depending on which cleat you put in which shoe. I believe the cleat with the dots goes on the right foot to obtain more float, but you might want to double check. It should say in the box your pedals came in.

Also, crank brothers cleats have a notorious break in period where entry and exit is stiffer than usual. That might be part of the problem.

Finally, On my shimano shoes I need to use the shims, but not on other shoes I've had. I would probably use them just to be sure nothing bad happens.

All but the first problem might also cause difficulty in release/entry. Are you experiencing that as well, or just lack of float?