If you move the CG behind the center of lift of the airfoil then you are requiring the horizontal stabilizer to help 'lift' that weight in order to fly level.

The slower you fly the less lift there is available from the horizontal stabilizer.

So when you are flying at typical flying speed (fast) the horizontal stabilizer is able to 'lift' the weight easily and you do not notice a problem.

At a slow landing speed the horizontal stabilizer is less able to to 'lift' the weight and you are experiencing the result of that reduction in horizontal stabilizer lift. Thus it is required to find some way to provide enough additional airspeed so that the horizontal stabilizer can provide enough lift to allow a controlled landing.

To exaggerate this imagine moving the CG back so far that it is now half way between the wing and the horizontal stabilizer. The wing will provide lift as usual but the horizontal stabilizer would need to provide a great amount of lift just to make the plane fly level.