Engine sprocket issue

Hi bit on a long shortly but.... I bought a track bike last year zx6r j1. Due to other commitments only managed to use it a couple of times last year. I've spent winter transforming it back.into a road bike and I have just put it back on the road and have had some small.issues... One of them which come apparent today when I had my MOT. I checked.my.engine sprocket last.week all look.ok.apart from nut was slightly loose. You could turn it left and right slightly With the lock.washer stopping.it.move any further. I tightened the nut.as per manufacturers spec and thought nothing of it. I've had to replace a number of part on the bike.and personally I thought the chain which was on was still ok but I replaced the rear sprocket for a renthal.


Cut a long story ethort I recently had the back.wheel.off . When I put the wheel back on. I adjusted the chain the play was meant to be 30 - 35mm it end up being about 25 - 28mm which I new was abit tight but i left it at that. Took the bike for its MOT this morning 5miles ride and when the guy was pushing the bike in the garage I noticed the chain engine sprocket was making an awful clunking noise and the tester advised that the chain was to tight and the front sprocket was on its way out. He adjusted the chain before I came home but wheN we pushed the bike out it made the same noise.returned home and when I pushed the bike in.the garage there was no awful clunking noise it had gone. Which I thought the adjustment which we made must of worked but then.I removed the engine.sprocket cover and noticed the nut was loose again of which I tightened last week. Why would that happen please advise as I'm going for a decent ride on Monday and abit confused why this is happening?????