As the title says, I have a drivetrain that persistently skips under any significant pedalling force, and I can't work out why. It's a singlespeed MTB with a swing-arm chain tensioner, the chain and sprocket (single cog on a Quando knockoff of a Shimano HG freehub) are brand new and the chainring has very few miles on it. The chain seems to be running fine on both the sprocket and chainring apart from the skips. I've also got the pulley on the tensioner lined up directly under the sprocket.

I don't believe the ratchet in the freehub is the issue, as I marked the spacers on the freehub body and the hub shell in such a way that the two marks were lined up, then rode the bike a few yards, being very careful not to coast. Despite more than one skip happening in that distance, the two marks were still lined up, and the freehub's not skipping far enough to have gone all the way round.

Any ideas?
Thanks in advance