Depends on the nature, more precisely, the chemical make-up of the stain.
Petroleum stains, like used motor oil or grease, are nearly impossible to remove from fabrics, whether seats or carpets.
Road tar or blacktop or blacktop sealers stains are nearly impossible to remove.
Red dye, like some wines or fruit drink, are also a real bugger to get out.
Inks from gel pens or ball-point pens or certain fountain pen permanent ink formulations or Sharpie markers are, well as the name implies, permanent.
Worst stains? Red Lipstick and red nail polish from women who put on make-up in their vehicle for work (I did not ask if she was driving at the time)
I`ve used Optimum`s Carpet/Fabric Cleaner and Protectant diluted 1:3 as a pretty good fabric cleaner for most food and coffee/tea (AKA tannins) stains for the most part.
Professional carpet cleaning chemical manufacturers will also have a stain-remover. I use DSC Products Inc 42068 Sure-Pass Heavy Duty Spotter I buy from a local carpet retailer/cleaning service for most concentrated organic stains. It has replaced Folex for me.
For tar and oil, I use WD40`s Spot Shot that you can buy over-the-counter in the laundry section at Walmart.
For human vomit or urine (kids throwing up or "accidents") I recommend Poorboy`s World Enzyme Stain and Odor Remover first followed by a carpet/fabric cleaner.
For the red dye stains, check a wine shop. They may sell a fabric cleaner for those red wine stains, but i think its more for clothes.
For winter salt stains on carpet floor mats and footwells, I use Sprayway`s Salt-Off for Carpets. It does not work as well as white vinegar and boiling water, but you do not have the vinegar smell lingering for three or four days.
I assume you are manually hand-cleaning with a brush and "extracting" with an absorbent microfiber clothe. Choosing the correct type of brush and microfiber is important. Seams in seats and piping or stitching require smaller brushes. I am cheap; I "re-purpose" old tooth brushes with a medium or hard bristle hardness (soft are too flexible, but great for applying rubber/vinyl protectant to door seal folds, transmission shifter boots, or wiring bellows!).
Carpet brushes require occasional cleaning themselves. Old carpet fibers and human hair (remember the women getting ready for work in the vehicle?) get embedded and I use a long,shout sewing needle to pick and comb out the embedded debris. Don`t laugh; it works.