The refrigerant that is used contains a lubricant, which helps prevent the seals from drying out, and a UV dye to assist locating leaks.
If the air con is not working, then it’s likely there is a leak in the system. Common causes are: seal dried out, failed weld (vibration/fatigue the root cause) or a stone/debris that punctures the condenser. Unless you can prove that the weld or component like the compressor has failed, then it will be very difficult (but not impossible) for a warranty claim - unless you can prove that it hasn’t worked since point of purchase.
With regards to can vs a/c recharge, any decent place should be removing the refrigerant (and subsequently weighing it to establish any losses) - if there is a significant loss then investigate where leaks are present. If no leaks, then the system will be placed under a vacuum in order to assist refill. Obviously, a can can not do all of this.