The ammo should not have failed to function because you kept it around for a couple of years or because it was stored in a safe.
So H&K4Life, all of this older HP ammo you had was only what was in your guns? You didn't have any leftovers, an old box, or anything to recall what type it was? That seems a bit strange especially since you are so emphatic on not having something like this happen to you again. I would have thought that the first step in not letting this happen to you again would be to avoid that brand and model of ammo.
As for not storing ammo for more than a month, that isn't going to do anything to help with ammo reliability unless of course the reason the ammo went bad was because of the primers and/or powder contaminated with an oil or solvent (as noted above).
Just curious, before you put up your guns, do you spray them with some oil and wipe off the excess or do you spray some in a rag and wipe it over the gun? If the former, then you may have ruined your own ammo.
Just curious, how did you come up with shooting 20% of a box before putting defense ammo into your guns? That seems a bit excessive. I am not suggesting it is wrong, just excessive.
If you purchased your ammo from Ammoman.com, can you recall if the ammo was LE only reclassified to practice ammo? Occasionally Eric gets that sort of ammo and markets it accordingly. However, the reclassified stuff I have purchased has been great. The ammo got reclassified (Speer Gold Dot) as I recall for some primer ignition issues. Apparently, something like 1 round in 1000 would be expected to fail. That was too high and so the ammo was reclassified.
I have had some for close to 4 years and it still shoots fine. Being 4 years old hasn't resulted in the ammo failing at a faster rate. With about 1200 rounds so far, the ammo has been 100% in igniting properly.
"If you look through your scope and see your shoe, aim higher."
-- said to me by my 11 year old daughter before going out for hogs 8/13/2011