I suspect there is a developing obstruction in the condenser of the A/C due to frozen condensate.
There is only 1 way to sort this and that is to contact the Service Manager and inform him of the problem. Tell him that you will bring the car in to him the next time it happens, for HIM to personally check that it does in fact happen. Once he has confirmed a problem, he will have to have the car properly inspected for an AC fault.
It may be inconvenient for you, but it will be the only way to prove that thee is a problem that needs attention.
I suspect last time, they turned it on and it worked, so they said, couldn't find a fault.
Please keep us informed on how your problem is attended to.