If the bedding is getting changed regularly, check the water bottle. Does it empty a lot faster than usual? Check for leaks, or if you have a dish then it could very easily be getting spilled. In the absence of anything wrong with the water bottle or the possibility of any outside source of moisture, change the bedding more often and more thoroughly. Hamsters with dirty cages are much more prone to health issues, and any problems they might have are a lot more difficult to diagnose if they're not in a clean environment.