It depends on alot of factors, but I have noticed this from time to time even in a humid environment.
Size of mason jar/how full it is
How often you open the jar
***Being the most important. If your in a dry/arid environment, then every time you open the jar your letting dry air into it.
I would get a bunch of small, jam size mason jars and put them into a gallon zip lock bag, or a larger, sealable container. This will insure a double seal in case the jars are allowing a small amount of air inside. But a properly sealed mason jar should suffice. I'd say opening and closing too often is the case.
Edited by toastmasta, 15 August 2013 - 07:33 PM.