Do you have the little ell tab on the inside of the mag or the outside? The tab is supposed to go on the outside. Also: Is the steel plate sitting flat on the bottom of the mag and not at an angle?
If you have the tab on the outside, and the extension can slide forward, the tab is either too short, or the little bump on the bottom is not locked into the mag.
If the ell tab is too short, call KT service 1-800-515-9983. explain the problem and they will send you free parts to fix it.
Sometimes though, it's just the stiff spring. Load it up with 6 and leave it overnight. See if it will take seven after that. There also is sometimes a problem with plastic flash on the follower that causes the follower to catch on the extension.
These are great when you get them working properly. No more problems with my two in over 2 years of use. And with the KT lifetime guarantee, If they broke even now they would replace them.
Packer.