I spoke to a gentleman about such things at woodcraft, and his reply was sensible and logical:
Oxygen. I'm going to assume you have a void at the top of your can now that you've used it, and of course air will fill that void, which contains oxygen, which will help "cure" your stuff sonner than you'd like.
The answer to the problem he gave me was 2 methods. Either put the remainder of your product in a can with no void at the top after use (switching cans 10 times doesn't make sense to me) or you can get a can of Bloxygen:
http://www.woodcraft.com/Catalog/ProductPage.aspx?prodid=9499 I'm not 100% sure that's your problem, but it sounds like it. Good luck.