Have you tested it to see if that's the problem?
How big of a jar can you get at Costco? I can only find the little jars around here. I do keep it in the fridge after opening but use it up pretty fast anyway. We'd have to drive about 90 minutes to get to a Costco but might be worth it if we had to pick up other stuff too.
Costco in Woodland sells the 32oz size of Red Star instant yeast. It is in a foil container. It is good yeast!