If you are doing the basics properly, and your loops are simply not maintaining their size, or are coming apart in the V where the bundles are laid back onto themselves as the loop is formed, try this...
Just before you lay a bundle back onto itself(same color) to begin forming the loop, untwist both parts, apply a little wax to each, lay them together butted tightly where you'd like the loop to come together, pinch it there hard, and smash/rub the two bundles together hard and fast to melt the wax and bring them together 'making them one', THEN twist them tightly for a few inches, but especially where they came together, before moving on to do the same thing to the others.
If you do this the same for each color, then twist and lay over tightly as you work away from the loop, it will be built sturdy and not open up. If your issue was something else, well nevermind then