trace

I have had this problem multiple times and I have no idea what's going on...

I'll ride my fatbike (a cheap one from Bikes Direct, has 4.9" tires with tubes) and everything seems fine, the tires are still full of air at the end of the ride. The next day, one of them is flat . When I remove the tube and inflate it to find a hole, there is no hole that I can see or hear. The tube inflates and holds air. I'll usually put in a new tube anyway (thinking there was a tiny puncture too small to detect, or a problem with the valve.) I inflate the tires and everything seems fine. If the bike sits around in the garage for a while, I'll check the tires every day and they are still full of air. But the day after I ride it, one is likely to be flat again, again with no detectable puncture and tubes that seem to hold air perfectly well. This happens if I'm riding at low pressures on sand, or higher pressures on dirt.

Also, this may or may not be related, but the tires are really loose on the rim. They almost seem too large for the rim (they are the stock tires.) Not sure if this is just how super-wide tires always fit. When inflating the tubes, it's difficult to get the tire seated correctly and requires multiple rounds of deflating and reinflating while reseating the spots that have popped off the rim.