In a large drive-in -10 freezer with dual evaps, the west evap only has frost on the top third of the coil after 5 hours (the east evap is frosted evenly) and the superheat is high, around 30 degrees measured just outside the box. It's definitely being underfed. I can't get up to the evap to study it without them removing tons of product, but if the TXV bulb has ice buildup around it could that cause it to underfeed? I would think that might only cause it to react slowly. The only other thing I can think of is the TXV sensing bulb has lost part of its charge.
It seems to feed really well when it first comes out of defrost, judging by watching the suction pressure. The box comes down to around 5 degrees in 30-60 minutes or so, and then the liquid feed just seems to taper off. As the freezer starts warming up to around 12-13 degrees, the superheat continues to climb and compressors keep unloading because the suction pressure drops off slowly. It almost seems like moisture freezes in the TXV but I don't think that's the case because how would it ever thaw out with electric defrost? The east evap works like a charm.
I wish I could get more info but they have food stacked up to the evap and because it's staying frozen they don't want me to mess with it. I guess they'd rather wait until it's an emergency.