I've had the following problems:
Weak spring that would not load when 3 or less rounds were in the magazine.
Slightly squished magazine that would trap the follower.
Feed lips closed too much...cartridge low in magazine at the front leading to jams.
Feed lips opened up too much...rear of cartridge sitting too low for the bolt to strip the round.
Also have had situations where followers and springs would simply not work with one magazine, but would with another.