I'm having trouble with one mag failing to lock my bolt back. It's an otherwise smooth feeding alloy follower USGI 20rd mag.
The bolt holds open on the last round with all other magazines,even crappy Orlites. This magazine will not hold the bolt, even when the bolt/carrier is manually retracted.
Is spring tension usually the reason for a mag failing to hold the bolt,or is it lip/follower distortion?

Spring prolly, but i have one that won't with different aluminum followers and new wolf spring.???

Is it one of Wolff's +15% type springs? How does it feed.
The thing is, this magazine feeds flawlessly and is very slick. It might just be that the bolt catch itself is just a bit too stiff in it's movement. It is in a new rifle build after all. I tested the rifle with about 10 magazines last night,including 2 other USGI 20's. One 20 was iffy,but the other was fine.
Ah well, it's destined to be a range mag I guess.

I bought a used mag made in 1966. Bolt would not hold open after last round.  I pulled out spring and stretched it by pulling it by hand.  Put it back in and it works fine.

Sounds like you need a new magazine spring.