|One thought is that eco-friendly detergents can sometimes act a little more like a soap, and thus be a bit prone to leave behind something like soap-scum. You could try soaking the diapers in very hot water and a lot of vinegar (if you have a top-loader, this is pretty easy: pour in a boiling kettle or stockpot to make the water even hotter, then let it sit and maybe just have it agitate once in a while). I don't know that will be a magic solution, but vinegar is considered a natural fabric softener and its acidity can also help break up soap scum. It really can't hurt anything, so might be worth a try. Just make sure you rinse them well afterwards.