Cannot find A/C leak

Where have you looked?

I would say that the most common leaks that I have seen are from the A/C condenser where the fins might have gotten damaged from hitting something, from the various valves/seals/o-rings throughout the piping, or from the compressor itself. Personally I have seen several with the leak in the piping or the compressor leaking from the main seal.

Also, when using a leak detector, you do have to use a UV light and glasses to see the leak. I did not know if you knew that or not, but most of those leak detectors require the light and glasses.