hmm.. i'm not an expert... but my air-cond has failed me many times... hahaha.. so from experience.. few things can cause your aircond gas to leak, therefore u won't be feeling the cold air.
1. Cooling Coil (Leak) most common problem Cost from RM190 - RM400 depends on cars. An aircond technician will have to take the cooling coil out and test if it is leaking.. he will den show it to you (the customer) proving that it's leaking. Otherwise, without testing.. it's bullshit.
2. Condensor (Leak or blocked) condenser unit is usually located outside the car in front of the radiotor. Typical life span for condensor is 5-8yrs.. thats it...
3. Air-Con Filter (rarely spoil) The unit looks like a bomb just right in your engine bay. RM30 to replace
4. Air Con Piping (rarely spoil) Unless abused or wrong installation process (expensive to replace)
5. Compressor (expensive unit) due to insufficient oil or insufficient air con gas.... overheating is another problem.
6. In some cars.. the air cond is controlled by electronics... So.. maybe the temp sensor is out of order... or the control unit is dead... (expensive unit for huge cars... and it's difficult to buy at times)
good luck mate...