I want to second what Melanie says. From my experience the ABC is one of the most versatile types of carriers (as far as fitting body types, being comfy for mom and baby and ease of use) and works wonderfully for *most* people. But I don't think there is any one sling or carrier that works perfectly for everyone. I have tons of suggestions to make them work but if you have already tried them all, then you may just be one of those people for whom they just don't "work" well and that is OK. Also, some people find they don't work when their baby is young but do when they get older and vice/versa.
Ya, you don't often hear of someone saying that their abc isn't working. But take comfort in the fact that it does happen from time to time, you are not the only one