When the freon gets low - 'cause of a small leak somewhere, it can do that.
What happens if it's got a low charge of freon there is enough liquid freon to compress and circulate - get cold, but as it circulates it's spread throughout the entire system, condenser, evaporator and everything else. The compressor starts running out of liquid freon to pump. Pressure drops, then the low pressure switch senses it's low on liquid, and cuts off the clutch.
Once it's shuts off, the freon liquefies again, pressure rises enough to satisfy the switch, compressor restarts - to do it all over again. It cycles like that.
So the effect is it's cool for a few moments, then stops working.
Icing can be another cause, but unless someone's had the system open it's not likely that.
Try adding 1 can - no more. Put a thermometer in a vent and watch the temp. Doors and windows closed. You won't be able to get enough in unless you have the engine running. Run at idle, with the freon can right side up - valve at the top - shake it every minute or so. The can will feel cold if it's feeding.
Take your time - the aftermarket do-it-yourself kits have a very small orifice - for your protection. With a larger orifice turning the can upside down could hydraulic the compressor, ruining it.
Always add it to the suction side - never the high side. If you are using one of those "do it yourself" kits it will only attach to the low side anyway.
You should see the temp drop down. Watch carefully - the temp will drop as you add - but when it stops going down and starts back up STOP ADDING.
It may take a second can to hit that lowest spot.
You may have a small leak and have to add a little every so often.
Harbor Freight - and Ebay, has nice little electronic detectors for finding freon leaks - about $30.
Using a pressure gauge with the engine off tells very little, -- it just says there is something in it - enough? Who knows.
The engine must be running and the compressor clutch engaged to mean much.