From what I've heard the button in the head is made of Aluminum and it's a softer metal then the steel glow plug, so it's easy for it to be stripped out. Even while unscrewing the glow plug I always take my time and make sure I do it straight, and a little squirt of WD-40 before removing the plug helps clean the dirt away and keep the threads clean (thanks for the tip TRD) so when you go to re-install the plug it's easier to feel the threads and not cross-thread it.