When the camera does not focus it is often a hardware issue caused by the PTZ cable or the camera unit itself. It seems odd that the issue persists even when you switch another one over. A few things to consider would be making sure that your drone is set to autofocus mode, making sure all the cables are making contact correctly, and checking the cable that runs from the gimbal to the main core although that gets damaged much less often. Another thing to check would be making sure you don't have any firmware incompatibility popups. Feel free to attach some screenshots or pictures and I am happy to check it out for you.
-Kale