What is wrong with my VDG?

i have just finished my small van de graaf generator but have run into a problem. I am only really building this small one as a model for my future large VDG so it is not to important, I would just like to find out more. My setup consists of two wooden dowl rollers built up with tape to "bevel" them. the bottom roller is coated with paper while the top is coated with teflon tape. the belt is a section of rubber balloon. the tube is a PVC pipe which is split so i can easly open it up and make ajustments. i have found this setup to be very good and can often achieve 2 CM sparks whith only a beer can as a top load. The problem is after about 5 miniunts the output will slowly decline untill I can no loger get any sparks. when this happens i have to open up the PVC coulom and clean the belt and rollers with an alcohol wipe. After wards, the machine continues creating 2 CM sparks until i have to clean it off again. I do understand the fact that dirt will be attracted to it and that enough dirt can bleed the voltage away but i do not think this is the case as i clean it so often and it is all sealed up. I just wanted your feed back because it is becoming tedius and i am starting to wonder weather it will affect the larger van de graaf.

Thanks In Advance