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The expansion valve or receiver drier restricted, so refrigerant cannot circulate properly. The expansion valve is the metering point of the whole system — it drops pressure and lets refrigerant boil — and the drier holds the desiccant that keeps moisture out.

A blocked expansion valve gives poor cooling with an odd pressure signature: high side high and low side very low, because the restriction is between them. Frost at the valve itself is a classic sign. The drier saturates with moisture once the system has been opened, which is why it is replaced whenever the system is, and a saturated one forms ice at the valve that blocks intermittently — cooling that works, stops, and returns after standing.