Working on manitowoc B 250 water cooled ice machince. Makes ice in freeze cycle,goes into defrost but ice does not drop.Put level on machine and tilted to front just little. Ice pattern looks good, hot gas valve is opening at least a litte because line at bottom of evap.plate is warming up. Can a dirty evaporator plate cause this problem ? Although it looks ok. Or can the hot gas valve not open all the way ? Can I check valve by measuring temp on both sides of valve ? Didn't have much time today to really check out going back tomarrow morning.