The only way to tell if it is low on refrigerant is to check it. Assuming it is not low because a leak was repaired is not the way to trouble shoot it.
Verify the work that was done is not the problem and then ask what else can be wrong.
When the a/c is called for the compressor should run until it is turned off, unless it has something telling it not to run. There are several things that can do this, low refrigerant is one of them.
When checking the refrigerant level make sure the RPM of the motor is high, simulating driving down the road, and then let the motor idle and the level should stay close to the same in both situations.
If it goes down when the car is idling, it can go low enough to trip a low pressure condition, causing the compressor to stop.
Also make sure the high pressure is not excessive, both readings are dependent on the pressure and temperature of the refrigerant. A high temperature can cause errors in readings. This can be caused by the fans on the radiator not running correctly.
Start with the easy one, check the refrigerant, first.
