Things that cause straps to loosen up:
1) Tieing to the body. Eventually, you hit a big enough bump that it slacks them.
2) Not locking the ratchets. They must be fully closed to be locked.
3) Not tieing at opposite angles. The straps should all be pulling in opposite directions. Best is if you picture 4 corners of the truck, tie each corner to the corner of the trailer. This way, it is secure in all directions. Straps that just go forward allow for side-side motion that can create slack. Especially if the straps are 100% straight.
4) Putting the straps against something that can move. Sometimes straps get hung up on a tire tread or something. Bouncing around sometimes makes the strap work its way over that tread and slacken up.
Best thing is 4 straps. Put an axle sling for each, around the axle tube close to each tire(don't crush your brake lines!). Then your strap hooks the axle sling to the corner of your trailer. Thats about as stable as you can get. You can use another small strap from your body to your axle if you want to compress the suspension. some people do, I never have any problem with sway..