Flexible PVC pipe was used for venting a High Efficiency Gas Furnace - Water spot in base of Chimne

While the High Efficiency Gas Furnace is being used a water spot forms at the base of the Flue chimney where the Flexible PVC pipe vents to the roof. There is not a stainless steel liner in the chimney. The furnace has two pipes going up the chimney, one for the in fluent and one for the exhaust. Rigid PVC pipes were used to exit the furnace and connected to flexible black PVC, with purple glue. The flexible PVC pipe goes up the chimney then is connected to rigid PVC pipes, at the top, with the purple glue, into terminates, consistent with a wall install. What could the water spot be? How do I test it? What should be done to fix the problem?