My guess would be that your chain slider at the front of the swingarm is worn out or missing you may also have too much slcak in the chain. I would also look for worn sprockets. there isn't really a chain tentioner, but I would theck the axle carrier bearing nuts to make sure they are tight, if they work loose your chain will become loose. You want about 1 3/8 - 1/3/4 or slack maesured at the bottom center of the chain.
Also, don't they sometimes lay on top of the chain slider until you are sitting on the 4-wheeler and putting some weight on it? I could be wrong, but I was thinking that was the case. Since mine has a driveshaft, it's been a long time since I have dealt with chains. My memory is not what it used to be. I used to have a memory like an elephant, but now I can't remember what an elephant looks like. LOL.