Jen this is soooooo common in bottle babies! You have done nothing to fail them. They just have more sensitive tummies. Try them on some yogurt and pumpkin too.
We are dealing with this at the shelter right now and all fecals are clear but for some reason we cant get it under control. I am dealing out lots of pumpkin and yogurt for nightly treats and its helping. Most babies like the yogurt.

Good luck with the little guys and hopefully they will be smelling fresh again soon!