Temperature ought not be an issue. Think of the millions of cartridges stored in Iraq, etc... heated to well over the temperature cited as a routine event. It's not that.
There is something going on that is specific to what you are doing. The *slightest* oil on the primer compound will do that... are you handling them with oily fingers? In fact, are you handling them at all? Best practice is to never need to handle them with your fingers at all. Correct tools help.