Matt,
The magazine base catch is held in place by the magazine base catch screw, secured from the inside of the butt plate. On my rifle there is a little bit of verticle movement when the screw is loosened.
Maybe your screw is loose (nothing personal here intended). If so, move the catch so that it is in the full upward position and retighten the screw.
If the catch hasn't moved down, try enlarging the hole into a slot so that the catch can move further upward to engage the slot in the magazine tube follower.
Note that there is a flat machined on the back of the magazine tube so that when it is rotated 90 degrees to remove it, this flat will clear the the catch. If you move the catch up too far you may have to remove some from this flat area.
Give these suggestions a try and if there is still a problem send me a PM and we can go from there.
HHW