An airplane has an aerodynamic center. It's called the neutral point and it's where all the aero forces equal out. Think of it as the hinge in a playground teeter-totter. An airplane actually is a teeter-totter that has aero considerations on both sides AND weight considerations on both sides of the teeter-totter.

As the CG moves forward from a balanced location, the horizontal tail is called on to carry that force. It is done with elevator trim. But it's a load on the elevator no matter what. And the elevator has only so much aerodynamic strength. So moving the CG away from the NP actually takes some of the elevator's strength and that winds up reducing the elevator's ability to steer the airplane in pitch. And as you move the CG farther forward, the elevator becomes less and less able to pitch the airplane. So your model feels more and more sluggish in response to your elevator input from your TX stick. Most inexperienced flyers interpret this as stability. They think their airplane is becoming more stable as the CG is moved forward. Actually, the elevator is simply losing authority.

Move the CG forward and the weight represented by that CG is down on that side of the teeter-totter. And the tail has to push down on it's side of the teeter-totter to keep the thing level. And in actual fact, with the CG force downward and the tail force directed downward, there is a real load created that the wing is going to have to carry. Most model flyers can't tell it's there, but the wing knows it's there. And the airplane flies with even less "agility".

Move the CG aft from a nicely balanced location, and the elevator gets more effective. Less and less throw is needed to pitch the airplane. Most inexperienced flyers interpret this as instability. The elevator actually needs less deflection to pitch the airplane, and often needs less than the throw it has at 100% of your TX stick movement. If the flyer would simply tune the elevator to move less on the airplane, he would have a more efficient flying airplane that wasn't elevator sensitive.

However, at some point in moving the CG back, the elevator becomes too sensitive to be controlled. Move the CG back even further, and the weight of the airplane has moved across the hinge point of the teeter-totter and it's force is in the same direction the elevator works. And that doesn't work for anyone.