Creating the cartridge

Hello. Can you put some screenshots and describe the issue with more detail so we can see what's going on and help out.

You may have double vertices causing the cut to be made on the wrong edges.
In edit mode select everything with A, then press M and choose by distance, see if that helps.
After making the cuts with the knife tool, be sure to press enter to confirm.
If not, like Omar said, share a screenshot and/or more info.

And the Knife Cut Tool cuts through Faces. If Faces are Hidden, for instance, you can't cut through them.

Like mentioned before, more info would be helpful.