Dew Point Issue for Instrument Air Compressor

Hi ! I work at a power plant and we are facing a problem with dew point. The required dew point is -40°C and the dew point remains around it but during Dryer's tower A last few minutes cycle it becomes positive. Tower B of the dryer works perfectly fine. We have regenerated the drying media in lab, also checked if any oil is being carried over and deposited over the silica but found nothing. We have kept the drains cracked open also to constantly remove water but still the dew point is fluctuating from -40°C to +15°C. What could be the cause of this behavior and what is the solution ?