There's a hanger on the rear of the magazine (metal tab that's bend over 180 degrees). There's also the mechanical retention on the front of the magazine. Both of these interact with shelves either built into the stock or that are part of the bottom metal (depending on magazine style). The rear metal tab on the magazine can sometimes break off which can result in the problem you are experiencing. It may also just need to be bent out a bit more to better catch on the hanger shelf for it.
Worst case scenario - call Savage and explain the issue and they'll likely mail you out a new magazine free of charge.
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