I built a Van de Graaf generator using a plan from Instructables with a few modifications.
I’m using two bed rollers, one covered in aluminum tape and the other with teflon tape. The rollers are 16 inches apart. I used an old bike inner tube as my belt. Its about two inches wide so it can carry a lot of electrons. My brushes are made of fine copper bristles that rub smoothly against the belt at both the bottom and top rollers. Oh, and I didn’t forget to ground the bottom brush.
The whole thing looks great and moves very smoothly but I don’t get any charge build up.
I am a teacher and have used Van de Graaf generators for years. I know they can be very fussy.
Any advice?
You do have a dome at the top for accumulating charge, right? I know it sounds obvious, but you didn’t mention it.
There are several Van de Graff generators on Instructables. Which one did you base yours on, and what changes did you make?
When I saw the thread title, I was thinking that this thread belonged in Cafe Society.
http://www.instructables.com/id/350kv-Van-de-Graaff-Generator/
I tried making the pin brushes that are suggested here but my experience is with wire brushes so I went with that. I used 3" ABS instead of PVC. I made my own belt instead of buying one. I have not connected it to a permanent motor just yet. I am using my drill to turn the belt. I used two aluminum bowls for a dome.
the bike inner tube may not be good rubber for this.
The bike inner tube may be too conductive - too much carbon black in the rubber.
My first thought was that the inner tube might not be ideal as well. Maybe get a latex exercise band and glue it into a loop.
Too late for ETA. Maybe an easily overlooked error. Is one of the brushes touching the belt?
both brushes are touching the belt.
I used Winsco brand Van De Graaffs for years. The belt installation instructions always showed that the brushes should be as near as possible to the belt without touching it. Also, to what have you grounded the bottom brush? On commercial models, it’s attached to the bottom casing. The casing is isolated from the motor and its wiring.
From your link, page 3, making the brushes.
The devil is in the details. 
Another too late for ETA: 
What about using metal nit combs for brushes? Pointy and even.
I have a working model from school (I am a teacher). It’s a smaller version. My goal is to make one much larger. In the smaller version, the brushes do touch the belt and it doesn’t work when the don’t. That is a detail that I have spent a lot of time with. I have spent hours making different types of brushes.
Did you remember to use the special evil laugh?
What was the belt made from in that version?
That belt was made from rubber but I’m learning that there are different kinds of rubber. Tire rubber might not be the best. I have just tried silk but it has no flexibility so it is hard to get the belt the exact right length. What I really need is neoprene rubber.