If you have an OLDER Kadet Kit (or Kadet) it could be the firing pin. They've changed the design.
I had intermittent problems with failures to fire until I got a new firing pin from CZ. I'm running a 15 or 16 lb. hammer spring (don't remember which, as I changed it out a long time ago.)
Also, cleaning the firing pin channel might be in order.
Talk with CZ-USA about it. Email SOMETIMES gets answered, phone calls generally do.
P.O. Box 171073
Kansas City, KS 66117-0073
Phone: (913) 321-1811
Fax: (913) 321-2251