You are either not cleaning down there properly, not wearing clothes with enough ventilation down there, or have a medical problem.
Are you circumsized? If not, it takes a little more effort to clean down there and it needs to be done regularly, or every time you get dirty/sweaty.
If you are, then you might not have good enough hygeine and need to address that problem.
eww...sounds like SMEGMA...
look it up