New to fixed/ss, have a bit of a problem
 

Hey, I'm new to the world of Fixed gear and singlespeed bicycles (and the forum-I posted an introduction thread a little while ago). I have a new Jamis Beatnik that I'm currently running with a freewheel, and although I really like it, I've noticed a bit of a problem with the chain. after every ride I take on this bike, it seems like I have to flip the bike over and re-tension the chain, as it seems to become slack over a course of time. My question is, what could the causes of this problem be, and how can I fix them? The bolts on either side of the dropouts are tight enough so that the wheel doesn't slip, so I don't think thats the problem. Insight would be much obliged.

The bolts are slipping, that's pretty much the only way your chain is going to be losing tension (unless you're pedalling so hard that you're bending the frame so the bottom bracket is moving back towards the rear cog which is... impossible).

Get a decently sized wrench (about a foot long or so) and get the nuts very very tight