I buy a MArlin 780 rifle in excellent conditions (8.5/10). The rifle comes without magazines. I buy 2 new magazines but the rifle cant hold the magazines. I dont know if the rifle have problems to hold them or if I need to buy an extra part for the magazines...
Without seeing the rifle, or having a very detailed description of what is happening (and not) with the magazine / gun interface I don't think there is much we can do to help you with this.