Air choking in Refrigerant Air Dryer

Dear I observe some problems in Air Dryer refrigerant type.I have a Comp Air dryer. In which a Hot gas bypass regulator valve is also installed. the problem is that air does not pass from dryer evaporator after some time ( may be after 5 hr or half day) it feel that air passage is closed.what happened ...... Also pressure drops after dryer outlet side. Also when we bypass the air from dryer (with out using dryer) then the compressed Air pressure of the line meet the requirement.