Swimming pool heat pump

Hi all, needing some advice. We have been asked to look at a swimming pool heat pump which is under warranty. it has worked well for the past 5 months but has recently started icing up on the evaporator. The heat pump is located within the pool building (around 18c today) and expels its air through a short duct to the outside.
Refrigerant is R410a, suction pressure around 6 bar, high side around 18 bar. Suspected system was short of gas, recovered and weighed it, not short, recharged it. When running the system the whole evap begins to frost, which is no surprise I suppose as its evaporating below zero but we were unable to get the pressure up.
We spoke to an engineer who does work for the supplier who said they had had some bother with incorrectly set expansion valves, after adjusting the valve there was still little difference to evap pressure. Exposed the valve bulb and heated it and the valve clearly modulates.

Any ideas guys?

Thanks

check coils are clean - get nitro on em- checkur superheat, any dampers that are operating correctly and not slippin on shaft, also filters clean on air inlets- txv bulb insulated

Cheers install, evap coil is as clear as a bell, no dampers involved and no filters. Bulb is insulated, opened valve to try and increase evap pressure but it made no difference, however valve does modulate. Do you think it could be to do with the valve?

check ur superheat/subcooling

I should have said, heat pump is heating the pool water, it is a self contained unit located within the building so air passing through the evap is at around 18c, this air is then expelled out of the building.

lower the pool water setpoint so it reverts to heating the air- see if the pressures change- usually leaks onthe o ring on a danfoss solonoid

Evap fan running? Properly?

Airflow into/out of evap restricted? Leaves, grass, paper?

has the unit got a defrost cycle

the evap unit in the room is the room pressurised or is the air blowing back into the room ( is there airflow through the evap)?

I wonder if the duct to the outside of the building could be enough to restrict air flow through the evap. Not been back yet, but think I will remove this before going any further.

check the outlet grilles as they can get ****ted up due to bird mesh/insect screens

They were clear install, I can't think of any other way to get the pressure up other than restricting the water flow. The heat pump is only there to maintain water temp. There's an oil boiler there to bring it up to temp.

Water temp in/out
air temp in/out
subcool
superheat
what water temp is it maintaining at the moment, has the water flow rate changed, ie pool not heating quick enough so pump speed upped? Is there a cover for the pool? is is a Calorex unit?

al