Been thinking and wondering how full the case is with powder. Have read that to slow a powder will over fill the case and when you seat the bullet it will compress the powder. Leave it sit awhile and the powder will move the bullet back out itself by moving to remove the pressure! I have personnaly never had that happen but then I always load to less than 100% capacity! Look in a charged case and if the powder is right to the intersection of the neck and body, try tapping the side of the case with a pencil a bit. if your using a stick powder it will settle the powder in the csae some and lower it in the case. If the powder is going past the intersection of the shoulder and the neck you are probably compressing the powder and that may be the problem. Load to the powder so that is does not fill the case beyond the junction of the body and the shoulder and that could eliminate the problem.
The problem is simple, sounds to me like the bullet is moving after being loaded. The answer just may be a case too full of powder! Seem's everything I've read about this it generally only happen's with a powder that is to slow for the case you are using. Powder relieves pressure on itself and push's the bullet into the lands, might not chamber!