You likely don't have enough charge in there, or it is overcharged (less likely).
When the compressor turns on, watch your gauge, and see if the suction side drops; if it gets below around 20 or 25 (somewhere around there), the compressor will shut off due to low pressure. While the compressor is on, you should be able to get enough freon in there to get the charge high enough to keep the compressor running, then get the proper pressure.
Mike -- 2005 "The Beast" (Formerly a V6, now a forged/stroked 5.0L V8 sportn' a Kenne Bell 2.6L @18psi, and many mods
2006 4.0L w/Powerhouse turbo and TR-6060 :kooky: