Your drop leg is way too far down your leg. Loosen up the straps that wrap around your leg so that the top one rides just under your nuts and the one below that fits wherever it hangs naturally. If it's tight enough, the vertical strap really just stops the holster from shifting back and forth, and thus down. Then, tighten up your belt a little bit so that it can't sit around your waist and sag that much. It's a belt, it's not meant to sag like jeans in the ghetto. And then, as Rabbit said, tighten up your suspenders to take up any slack.
Here's a good example of where a drop leg should hang: