How do you prime your cases? Any chance you handle primers one by one? Could you have sizing lube, hand lotion, other, on your fingers when you handle primers?
Any other opportunity to contaminate your primers? How / where were they stored?
I'm currently using primers I bought in 1993 and maybe some from in the 80's. No problems. One reason I use the Lee Auto Prime, so my fingers never touch the primers. I did contaminate some a long time ago - was a pain. Lucky I loaded in small batches.