This doesn't make sense, the aircon either works or it doesn't.
If it doesn't they need to tell you why so that it can be repaired under warranty surely.
If they say there is no fault what reason do they give for it not working?
Surely it shouldn't need re-gassing after just 2.5 years?
As a matter of interest, which model Dynamique is your car? Have you owned it from new and carried out services as required? Do you use the aircon regularly or have it turned off most of the time?