Vacuuming sand requires a completely different technique than gravel. What are you using to vacuum? I use just the clear hose without the gravel vac attachment at the end. After you get your siphon going, start with the hose a couple inches above the sand and slowly lower it down to the debris you are vacuuming just enough to suck it up. You can make little swirl movements with the hose to help stir up debris without getting too close to the sand. It takes practice, but once you get the hang of it, it becomes pretty easy.
I see that you have a power head in your tank. I would recommend setting up your HOB and power head so that all of the debris collects in one place, and you can vacuum it out very quickly and easily.