why can't my fridge be repaired?

The technical answer I received from my local caravan repair workshop was "Your fridge is dead mate, we've tried everything we know to resuscitate it but the coolant will not circulate".
When I realized the cost of installing a new fridge would be in excess of £500 I thought hang on a minute, if as they say the coolant will not circulate and I know they have tried every upside down trick in the book so why wont it work?, what actually is stopping it circulating?. The pipework all looks in good condition, I asked if it could be a pin hole causing loss of chemicals and the reply was you would know by the smell if there was. The fridge is an RM 4206, okay its getting on a bit, but age alone can not be the cause of death, something has to deteriorate, break down, or foul up. Any ideas?