When good glow plugs go bad

[pselaphid]

A while back I was whining about a no start/occasional start condition. All evidence pointed at glow plug failure. BUT, I had replaced them about a year and a half ago with, presumably, good Bosch France gps, along with the relay, so I had doubts and wondered if something else was up. Anyway, it was the gps and a postmortem revealed that only one of five was glowing, and maybe a second, intermittently. Its a wonder the thing started at all...on two plugs near as I can figure. I put new ones in and it starts within one second of cranking like it did before.

Question is, were they just poor quality plugs to have under a year and a half service life or is there some underlying reason why they go bad? I realize they live a hard life, but I can't believe this thing came off the line 25 yrs ago with gps that had to be replaced every couple of years, at max.

And a followup question, how soon should I plan on replacing the Autolites that I stuck in it because they were locally available (I had to go somewhere)?