Give full rudder, aileron and elevator. It should go into a fairly fast spin. Release the elevator and aileron and hold the rudder. If it doesn't stay in a flat spin then either the CG is too far forward or there's not enough control throw.

To come out of the spin, release the controls. If it continues to spin then give opposite rudder. If it still continues to spin then on the plane you replace this one with don't move the CG so far back.

Actually, if it keeps spinning, give it opposite aileron. If it still spins, give it a blip of throttle. By now if it hasn't come out of the spin, it's about a split second from impact.

Most planes that don't spin have a too far forward CG. Most planes that won't come out of spins have a too far aft CG. If you have to move the CG, do it a little at a time.