First off, I'm going to say too much defrost.
A case like that will do fine with 4 a day, although you might need to stretch them out to 35 minutes or so.
Those can be a royal pain to get to hold temp.
You MUST cover all of the bases. Get your superheat down, your suction pressure is a bit high for a case like that. I'd like to see an evap down closer to 15-18 rather than 21.