Are cardboxes different now compare from beginning (single wall to double wall card for example)? this can affect cooling speed of meat depend of higher isolation.

pressure 75 kPa absolute give boil temperature of -39 degree C, so is seems same as your accumulator temperature

for -21 degree C vapour pressure should be 181 kPa Abs so question is if you have restriction/valve between inlet to plate and platers inside pipework with over than 1 Bar pressure drop and question, actual working pressure in pipework?

have you measure surface temperaturer on plate ? - this should give idea if plate work around -21 degree C temperature and restriction for vapour is near end of plate or if work around -39 degree C, restriction is beginning of pipework