Retained placental fragments - Antibiotics. Hard, heavy, and fast. Inspect the placenta, if not 100000000% sure its ALL there and is a nice pretty healthy looking one or even a touch stinky, call the vet.
Infection - Same as above. Often called 'metritis.' I like uterine flushes and packing with various treatments as support. Van Beek makes an AWESOME product for cattle with tea tree oils that is put right in the uterus via capsules. They smell minty! I think thats 10X more effective than antibiotics! There has to be a comparable product for horses, but, only VERY experienced people should pack or flush a uterus. You can do a TON of damage if not done right or not needed.
Untreated infection - Antibiotics. Harder and heavier. Will keep her comfortable, but I can just about guarantee she won't ever breed back. If left untreated, adhesion's, scarring, etc occurs which is irreversible.
Scarring - No cure, will NEVER be fixable. Same with adhesion's.
Ovarian cysts - What type of cyst? In a luteal cyst, a super shot of prostaglandin to regress the CL, no CL, no cyst. These are harder to treat though. A follicular cyst can be cured with a supershot of GnRH. Makes her superovulate and stops the 'traffic jam' on her ovary.
Immature reproductive organs - How does this happen without a breeder being grossly irresponsible? Either the mare is too young to be bred or had her growth slowed from improper care.
Wind sucking - I have had zero luck in fixing this with my cattle, not as prevalent as with horses, but even a caslick stitch hasn't worked on the girls that do it. However, a lot of horse's seem to do well with a caslick. Not real experienced with horses on this one. Just doesn't seem worth it to me to produce more windsuckers....
Loss of uterine tone/poor uterine anatomy - Loss of tone means she isn't in heat. Even a 'well used' uterus will have a VERY palpable tone when she's at peak fertility. Poor uterine anatomy can't realistically be fixed.
Glandular disorders - Which ones? And why even breed a mare that has them? CULL.
Vd - Once again, responsible breeding. No cure for fertility - if the damage is done, its done. Shame on the breeder for letting it happen! This is why most other large animal industries have gone to AI. Safer, cleaner, WAY reduced chance of VD. However, equine AI is FAR behind the times!
Reproductive damage due to previous dystocia - What kind? Torn uterus that heals well? C-Section? I've had no trouble getting cattle bred back (even OLD ones! one was north of 10 years old!) with this issue if there's no infection. Episiotomy? Those need to be stitched RIGHT away, if not its just cruel to even do one. The issue is when dystocia is left untreated, it becomes big and inflamed, and she heals on her own. The WORST thing that can happen in a dystocia is to have someone that doesn't know what they're doing fix it. Hooves, elbows, knees, and teeth can all pop a hole in a uterus easily! I will spare you my rant on the equine industry about dystocia. With proper breeding, proper prenatal care, proper mare conformation, and education there is no excuse for it to be as routine as it seems to be.
Hope this helps. Large animal repro is a HUGE part of my job. I routinely AI cattle (8 or so a day? give or take) and deliver of hard to pull babies, and care for the mommas! A dirty uterus is a dirty uterus though, and treatments are surprisingly close.
My opinion on infertility is that its usually our fault, sometimes a higher power intervening. Either she received improper care at some point that destroyed her uterine environment, or a higher power is intervening and keeping her from passing on her infertility to the gene pool to make more infertile animals. I can live with an animal coming up now and again with something 'just plain weird' happening in her lady parts, but she is culled, and i put no effort into getting her bred because i don't want more infertile animals!