Chain Tensioner problems.

Assuming the entire tensioner isn't moving, that your build is still comparatively new, and that your rear wheel isn't shifting in the dropouts - I'd hafta guess it's simply sprocket wear. Often called "chain stretch" (not entirely inaccurate but also not primary), it takes some time for the chain to seat completely in both sprockets - for all three components to wear together and "match" properly.

Considering the... less than perfect manufacture of all three - it often takes far longer than you'd expect, sometimes seemingly never ending as is the case with my setup. I replaced my chain with a 410 well over 2500 miles ago, though it's not nearly as drastic as it was initially - I still suffer from what I consider "undue stretch" and need to keep a watchful eye on it.

Slightly off-topic but still relevant, have you bent the tensioner to be inline with the chain and not the dropout? I only ask as this is often overlooked... there's much advice regarding the bedanged tensioner to be found here on the off chance you've not seen it: http://motorbicycling.com/f11/chain-tensioner-suggestions-11815.html

I hate the blasted thing, if any of the alternatives in that thread are suitable defo "get rid" of the stock setup - including actually getting rid of it entirely if your build allows it

If you take the flat washer out from the tensioner wheel & only use the lock washer it won't move as much!