I dont know the volvo system but most A/C systems have various protection systems built in, one of which is a pressure switch which will prevent the system working if the pressure in the system is too low. The reason is that the gas is mixed with a lubricant oil and if it runs without it then your compressor will seize up - expensive! This is also why it is recommended to use the A/C all year round, the lubricant oil stops the seals in the system from drying out. We often see cars that have been out of gas for a while and gone to a cheap place that just puts gas in (often without any oil), the system seems to work for a short while then fails, the seals have dried out and the regas just blows through.

Depending on the system design, low presure can start cold then go warm, or take time to get cold.

Go to an A/C specialist and have them check the pressures, especially if its a while since you had a regas.

If you are on R12 and that is no longer available in your locality ask about RS24. R134 isnt always compatible with older systems.

Mark