1. Your car's condenser fan may not be working, or at least not correctly. With the A/C on MAX (car running) open the hood and see if the condenser fan is working. If not, you have found your problem already.
2. Your car could be overheating and you are not aware of it. If your engine is running too hot it will definitely effect the A/C... especially at idle.
3. If your condenser is clogged with debris, this will cause a decrease in the effectiveness of the Air Conditioning System. Remember to check between the radiator and condenser for for this Debris as well.
4. If you are low on refrigerant this can cause a decrease in cooling while at idle.
5. If the desiccant bag has ruptured in the dryer or accumulator, this will stop the condenser up from the inside, until it can not do it's job. Lets hope this is not the case for you, because this is a major problem.
6 Finally, your compressor may be worn out and not pumping the refrigerant thru the system as needed. Again, this is a major problem as well. So you may want to consult your local Auto A/C Specialists if you have any doubts.
Kool Kar Auto Repair is the A/C Authority in Texas. Call 817-270-2549 to make sure your vehicles A/C will get you thru the remainder of the 100 degree plus days in Texas.