Yep, I agree with everyone else here - lots of variables to be examined before you can determine the cause. I do detect though that people seem to think that a mandrel saver is the only golden ticket to longevity.
I never use a mandrel saver, I always use a live centre in my tailstock. I replaced my mandrel in the autumn last year after 8 years of service from the previous one, on which I made 5 or 6 pens a day, 5 days a week, throughout its life. Do the maths and that is conservatively in the region of eight to ten thousand pens before I finally retired it.
So a mandrel saver isn't always necessary if you want your mandrel to last, but not applying too much pressure from your live centre is paramount.
However, you mention a dead centre, which may be a big part of the problem, and you also need to establish which of your two centres is not true, if not both.
