One other possibility (especially if you purchased the rifle used) is that the magazine lips are too close together and need to be tweaked just a bit to let the cartridges sit that much higher in the magazine.
Do you have a friend with a long action DBM? Perhaps you could try his or her magazine in your rifle and see if it works. If it works and yours does not, it could be that someone prior to you adjusted the feed lips.
Whatever you do, don't try to bend the little rear tab. They snap off. I'll leave it to you to guess how I know that.
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