What make is your replica?
Does it happen with all of your mags?
I previously had encountered the same issue with my GHK M4, turned out is was the Devil Hunter feedlips, that were by his own admission when I contacted Pablo
They we’re ever so slightly out of spec, and stopped the follower reaching the top
You should maybe stretch the follower spring a little to see if that helps, or shim it at the bottom to add a little more compression
Some folk put a bb in below the spring to do that
It'll reduce your ammo count by one bb, but should make the difference
Failing that, you could bend the tab downwards on the replica, the one that the magazine will lift if applicable as I don’t know what M4 platform your are pewing