Supermarket R407F issue

[NickNZ]

Hey guys, first poster here so I'm having a issue with a r22 retrofit supermarket I look after. The medium temp rack is having issues with evap coil starvation causing icing up and water leaking. Visual look of the sight glass shows a liquid vapor mix in a river, however the level indicators on the reciever show 1/3 to 1/2 full. So why is there gas in the mix?
Electronic compressor staging with 4x Copeland d6''s pressures set at 46psi and 200psi. I've noticed incresing the discharge setpoint up to 220psi or 240 results in a instant full sight glass and correct superheats.
However it doesn't last and the starving issue comes back again after a few days even with the higher pressure setpoints.
It's rather confusing, recently there has been a change to suction control solenoids rather than liquid line to reduce liquid hammering we were experiencing, but otherwise the system is the same it's always been since going r407f 6+years ago.

Has anyone possibly experienced this or similar?
Thanks
Nick

[seanf]

Whats the ambient temp, subcooling and differential of the condensor fans?
Where is the LL sight glass located?
Does a light and a heavy cooling load give the same flow in the sight glass?
Do short and long cooling cycles give the same flow?

[kiwifridgie]

Is sight glass after driers?
Check those for crap from the retrofit.
Seen it before.