Nerever,
The kit supplied #415 chain can give you some problems. Check it closely for binding and either free up those areas or replace the chain with a good quality industrial #41 chain. This chain is available at Grainger, Ace Hardware, Tractor Supply and other industrial suppliers. I've experienced rough spots in the kit chain the will cause the problem you're having. I've seen rollers that wouldn't roll and even a kink (twist) in the kit chain. Also make sure your chain tensioner, if you're using one, is tracking true and not allowing the chain to run off to one side or the other. Most of the chain tensioner brackets will require a slight twist to align it perfectly with the drive and driven sprockets. With the chain removed, spin your rear wheel and watch for any wobble in the rear sprocket. It must run true to keep the chain running smooth. Use the advice above to make sure your sprockets are running true and wobble free. Hope this helps.
Tom