Unfortunately, I've found that the stock Diamondback DB9 magazine springs lose some of their strength over a few months of use and a perfectly good magazine can all of a sudden become a dud. A couple of weeks ago I posted a blog entry about using a trimmed Kel Tec PF9 spring in place of the stock DB9 spring. You can read that here.
While it will take months to really see if this spring change is the ultimate solution to my feed problems, I did want to share what I have learned so far in case you find yourself in my situation and want to try the spring fix for yourself. My feed issues look like this. As the round is stripped from the top of the magazine, it will nose dive into the feed ramp and jam the pistol. This only happens with a full magazine of 6 rounds.
I shot a video of the work involved in replacing the stock Diamondback magazine spring with the Kel Tec PF9 magazine spring, which can be ordered here. After reading this and viewing the video, you may decide to try this for yourself. I've had this same feed problem with two different DB9s and the magazine spring change has resolved my issue with both DB9s. I can't assure you that this will fix your feed problems, but I can say that it helped me.
My how to video explaining the issue and converting a factory magazine spring to the Kel Tec spring follows.