Taking the covers off to clean can be both a good & bad thing. Oxyclean in COLD water! Hang to dry, NOT in the dryer!
If they are still not clean enough, you can try this. 'Zippo' type lighter fluid. It's naptha. Same as what dry cleaners use.
You can only do smallish areas at a time, because it evaporates fairly fast. Say, 8" X 8". Drizzle it on the area to be cleaned, let it sit for a minute or two, then using compressed air (blowgun) blow the dirt through the material from the face side. The naptha will loosen the dirt so that the air can push it through to the other side. Use the blowgun in a grid pattern. Side to side, corner to corner. Do the whole cover, overlapping the areas being cleaned with the naptha, completely once, before the second time, if that's needed. If the covers are of a lighter color, then 2X is the limit for doing this. The naptha has a bit of a yellow color to it and that will stain the material.
EXTINGUISH ALL FORMS OF FLAME in your well ventilated work area!!! DO NOT even think of putting the covers in the wash machine after this. Things will (apparently) go BOOM!
If you are uncomfortable with using the naptha, you could also use a household carpet cleaner in the same way, but work that into the covers first before using the compressed air. No bristley kinda brushes. Hands & gloves. You want to get the cleaner right into the material so it can do it's job of loosening up the dirt. If you aggressively scrub, all you're doing is pushing the dirt around, causing the dirt to abrade the fiber of the fabric. Let the cleaner loosen the dirt up, then blow it through with the air.
Automotive seat cover materials are, with exception of leather, 99.9% man-made fabrics. The reason for that is that those materials are tougher than anything from nature, & by design, tend to be resilient to chemicals. The fibers are so smooth that they will resist holding onto stains. It's the dirt that causes abrasion between the fibers that will break down the material.
