Is it the vinyl or poly cotton-like material?
I found Scotchguard yellowed over time, especially with exposure to sunlight, but it is effective as a barrier. I used Arm & Hammer baking soda and peroxide toothpaste. A dot of that on a white slightly moistened cloth or a soft toothbrush usually took care of small stains on both fabrics.
Or, do what squared-away officers do. Buy 3-4 at a time. Always have one inspection-ready and pristine and only used for inspections. Use the others in rotation, cleaning as you go, or just have a ready replacement. The mids at USNA used to wonder how all the senior officers managed to look fairly pristine in whites all the time. A peek inside my office closet would have revealed 5 pairs of white shoes, two fresh sets of whites on hangers, and a stash of fresh combo covers.