Looking at your photo, I would suspect you need more scope. You need at least three times the distance from your bow to the bottom. So if the water is 5' and your bow is 2' above the water, you need at least 21' of line out. Books say 5 to 7 times the distance but that's tough in a crowded sandbar and overkill for short term anchor.
The fluke anchors work pretty good but I've had better luck since I switched to a plow anchor. Mine is 22lbs on a 16,000 boat. Looks like this but a no-name brand. Also I have 25' of chain, it makes a huge difference.
