I've owned a CZ57B in the past.
If I remember correctly there is a piece of black spring steel that purposely rubs the magazines so that they (the mags) DO NOT drop free.
To "fix" this, you need to remove the grip panels. Then remove the hammer spring (forget how to do this... don't have a CZ in front of me to try.) Then you can get access to the black spring; this spring is held in place on top by a pin if I remember correctly which can just be pushed out - the bottom was held in place by the hammer spring assembly which you would have removed at this point.
Once you have the black-sping in hand you simply take the bend out of it by bending it in the opposite direction; you want it to be flat.
Voila... re-assemble and the mags will now drop free.
~Cheers and good luck
If you are mechanically challenged, you can just remove one of the grip panels and bend it back while installed (but doing it this way might not make it perfectly flat - no big deal really, but I am anal for things like this.)
hehe