Have you taken them apart and looked at the springs and followers? Also check the inside of the mag bodies for rust or debris that could be hanging them up.
I'm not sure what you mean by "bullet overhead". Do you mean clearance between the tip of the bullet and the forward wall of the mag? There isn't usually a lot of space there. What type of ammo are you using? A different manufacturer, or bullet weight or style(fmj, sp, hp) may make a difference.