Make sure the wheel is not hitting the mounting bolts. On all my tensioners if I have the wheel to its lowest point there is interference with the mounting bolts for the tensioner.
Remove the chain and keep your tensioner wheel where you have it set for proper chain tension and try to determine what is the cause.
Sometimes the wheel can be right against the bracket not allowing it to spin, especially if the tensioner had to be bent for a straight chain alignment.
Also, most tensioners will have a small collar to give enough clearance between the wheel and tensioner bracket. Without it when you tighten the bolt down you just end up locking the wheel against the mounting bracket.
Ok will try... usually all I have to do; instead of removing the chain is loosen the chain tensioner bracket and slide it towards the front of the bike, and it will loose tension, but I am going to keep it in the same position this time and
Also; I don't remember if I've mentioned to you that you can most likely remove the chain tensioner completely. It is usually not needed.
If your drive chain doesn't rub on your chainstay, you should consider getting rid of it.
Lot's of folks use them, but I see them as a potential problem that I can do away with. Think of that thing rolling over into your spokes while you're at speed- pretty bad news.
Sizing your chain appropriately will allow for this.
If you must have one, the best type bridges the chainstay and seatstay.
If you must use a tensioner you need to assure the tensioner wheel is aligned with the centerline of the chain. The kit tensioner bracket will need to be bent (twisted) to achieve proper wheel/chain alignment. As it comes in the kit the bracket will not allow the wheel to run true, centered on the chain.
But as Trey said, if you can run without a tensioner, do it. You'll want horizontal drop outs or some way to keep the chain properly tensioned. It will wear and loosen over time and the kit chains loosen quickly.