If you have done verything properly, the last thing I can think of is the expansion valve - it stuck open. In the old days, I was taught to test the valve by spraying r-12 on it and the low side pressure should drop to around zero. If it does not drop, replace the valve. Now, you cannot do this anymore for both economic and environmental reasons.
Retrace your charging procedures. Is it possible that air leaks into the system without you knowing it?