One example of a mixture that can become more sensitive with time is Dr. Shimizu's Napthalene black smoke star formula.
It contains chlorate, sulfur, and a large excess of napthalene as fuel. There's so much fuel in it that it's not even the least-bit sensitive to either impact or friction.
But....napthalene sublimes into the air at STP. So, after a time, the stars become fuel-poor, and the only primary constituents are chlorate and sulfur -- OOPS!
That's one of those formulae that needs a "best used by date" on it! <grin>