display chiller not getting cold enough

Hi, New here and couldn't find any ideas on this
I have an igloo chiller display cabinet
been working fine, but suddenly doesn't get cold enough
The fan is working on the condenser radiator and it is clean and not blocked.
Pipes into it hot, and out are cold
pump appears to be working as hot and slight vibration felt
but top condenser radiator part in chilled part, get frosted one end, but not cool other end
is there a usual reason this may happen?
I assume as the frost forms the coolant must be ok
temperature is set to 2 degrees, i have checked, but cabinet wont go below 9 degrees now

Sounds (or should that be reads) sort of charge/restricted. Needs a chap with know how to confirm.
Always worth blowing the condenser out with nitrogen / compressed air every so often as well.

Cheers,
Andy.

as said restriction or low charge, need to get a set of guages on it to know wich may be the case.