try searching back about 12-15 days or so, there was a thread on humidity affecting chalk, and it sort of turned into a what's-your-favorite-chalk thread. I imagine there are a few of these on the site.
If I remember right, an actual official from Tweeten Fibre Co. (the makers of Master chalk) swear that there is absolutely no difference between pre-flag and post-flag Master chalk, it's an urban legend.
It was also suggested to keep the stuff in tupperware if humidity is an issue and to not mix different flavors of chalk. Also try throwing in a few of those little silica packets that come in beef jerky and whatnot. They keep things more dry.