Help required with 2007 S3 Air Con / what am i missing here ?

Hi
I have a 2007 S3 and the air con is not working, we have tried everything, what else am I missing here ?
New compressor ( wiring all checked and working )
New Pressure switch ( wiring checked and kicks the compressor in )
New climate control unit installed in the dash ( all works )
Fuses checked all good
Relays checked all good
Re gassed with 525g of refrigerant.
And its still not working !

Its not even trying to start or work, has anyone got any idea why or is there something else I can try ?
any help would be much apriciated
cheers

All the work has been done at a friends garage, there are no leaks, belts, filters, etc all fine.
The initial problem was the compressor packed in, and it's never worked since.
It's not even trying to work
The regas has been done on a propa machine, full recovery, passed the vac test etc.
The garage don't know what to try next, so I am looking for other things for them to try
Cheers