Some things I've learned about industrial belt-drive systems over the years. Should apply to SC drives. Hopefully, it will help. Maybe not.
The belt itself is seldom the problem. (Assuming what you are using is reasonably close to original specs and it's not worn out.)
Pulley material/construction is just as important as with the belt. If the pulley doesn't provide some grip for the belt, you're going to have slippage. Get a belt/pulley combo with sufficient stiction and you can get a little sloppy in other areas.
Everything else being the same, pulley contact area will have the greatest effect. Contact area can be increased by having the belt wrap around more of the pulley. Contact area will be decreased if the belt doesn't run flat on the pulley.
Flexing of brackets and other mounting hardware should be kept to a minimum. Flexing causes loss of tension and misalignment of the belt and pulley. This was the cause of many problems I ran into.
Belts will naturally vibrate but if they start to flop, you need more tension or an idler.
Tension is important, but y'all already knew that.
