Lots of people on here swear by MTG but I have never used it. Unfortunately, much of a horse's hair quantity is determined by genetics. Some horses will always have skimpy manes regardless of how much care they get.
Try a good protein conditioner w/o silicones in it. Has done wonders for my horses hair and my own ;) (VO5 makes the one we use) Oh and if you must shampoo try to find some sulfate free ones. Sulfate really dries their hair, coat and skin and makes it brittle and dry. I save the shampoo for tough spots that don't want to come off (which we haven't had to use, the conditioner seems to wash everything off)
My mares mane just won't grow like my other mares!
Hers is just so short, choppy, and thin
The others is long, and thick.
Is there anything that can you do for it?
You know, an appaloosa will never have a mane like a haflinger. As smrobs said, lots is determined by genetics. You can try various conditioners and supplements but you won't unfortunately change the way your horse was born. If her hair is damaged, she's rubbing it off or has some kind of a skin problem you can treat those with success to get thicker and healthier hair (and longer in case of rubbing) but I'm afraid you can't change the amount of hair your horse grows.