As the system approaches full charge, it doesn't take gas as quickly. Also, the compressor has to be cycling in order for your ac system to pull gas from the can. It could take quite some time to get the job done. Check your pressure and vent temp. Pressure on the low side when the compressor is cycling should be 30-35 psi. On my system at that pressure and 80 degrees ambient temp. I can only get 56-58 degrees. If the ambient air temp. is high, you won't be able to achieve really low tmeps. One other thing...Make sure your system is on recirculate when charging and checking temps and remember that you will actually get slightly colder air when fan is not on high because you are moving less warm air over the condenser.