Yes you should be able to turn the centre of the wheel. The pulley that the belt runs on just free wheels unill the compressor clutch engages electrically.

Just because it was gassed last week doesn't mean that the gas hasn't leaked out. If it's got a leak, it can leak out in a few minutes, a few days or even a few weeks....

It could be that the pressure switch has failed or the clutch has gone open circuit or a wiring fault or it could be any sort of sensor. Even the old Escort non turbo diesel vans had sensors and diodes all over the place.