For leaks that cannot be found with UV dye, there are several additional steps. The first is to remove the nut on the front of the compressor and take off the clutch plate. You then shine your UV lamp to see if the shaft seal is leaking. It is not an uncommon leak and will often not show up with dye otherwise.

If that doesn't work, the next step is to find a shop that will recover the refrigerant, add 2 oz. of R22 then pressurize with nitrogen. They then use an electronic leak detector. R22 is MUCH more detectable by an electronic leak detector and this method will find any leak.

Good luck,