2007 Mini Cooper S Air con not working

Hi i have a mini cooper s that had recently has a air con refill. After 3-4 weeks there is no difference in air temperature may be very slightly when ac on or off. i hear a hissing noise in car cabin- centre of dashboard but cant hear it under the hood. took it back to kwik fit who said there was no loss in pressure but do not carryout repairs. The technician said it may be compressor (you cant tell by looking from the out side as there may be an internal failure) but it seems the cold air is prevented from being pumped around the ac system. I do hear something clicking when i turn ac on and the fan does run. Any ideas