Have a frigorex coca cola fridge in my shop, at the moment it is not cooling stock, what are the usual problems with this fridge

Fridges not cooling regardless of make suffer from several problems.
low gas from a leak door not closing properly/leaking door seals
compressor motor not running from low gas /overload/burnt out
faulty thermostat /settings. In a business because you need the unit to operate all the time ,get a fridge mechanic in to fix it