I am having real issues with yeast. For some reason, more often than not, it refuses to activate. When I first get some and use it right away it works but if its left in the house more than a couple of weeks it never works. This is fresh yeast, easy yeast and fast action yeast. I have no idea what could be killing it but I have boxes of yeast that has no use whatsoever. Any ideas what could be causing this?