Welcome to the Forum BA
With the Upper removed, but the Slide Stop installed fully; insert a KADET Magazine
Note how the Follower and the Slide Stop interact
The Follower should be "trying" to lift the Slide Stop
If the Slide Stop doesn't touch the Follower, the issue is the Slide Stop
If they touch, but the Follower doesn't lift the Stop, then it's a Spring issue
The KADET Mag Springs, are the Mag Spring .... nothing you can to do there
That leaves the Slide Stop Spring
You can try a couple things:
Insert the Slide Stop from the Right Side, just enough to get the end under the Spring
Rotate the Stop several times to "loosen" the Spring a bit
Sounds funny, but this works most times
OR, take a round File to the Spring Notch in the Long End of the Slide Stop
File the hard corner off of the rear of the Notch

Also, check-out this Thread:
https://czfirearms.us/index.php?topic=22581.0If all else fails .... call CZ-USA
Hope you get it working correctly soon