You may want to consult with a lactation expert. Yes, I know this isn't for breastfeeding (which according to you, your baby seems to know how to suckle). BUT, be aware that sucking from a breast and sucking from a bottle-nipple requires 2 DIFFERENT muscle coordination "techniques." Breastfeeding and nursing from a bottle are not the same. Bottle nipples use gravity... breastfeeding requires suckling. This may cause confusion in the baby.... and thus, inability to nurse from a bottle. I would recommend looking into this.
Also, make SURE that your baby is indeed suckling/breastfeeding well. Sometimes they are just "suckling" but not well enough to be taking in enough quantity or they are not latched on properly. This has happened to a couple of babies of some friends of mine.
On another note, I have used the "MAM" baby bottles for my baby when I had to wean from breast, and my baby had no problem with the transition. And, it is not made of the harmful plastics that contain BPA's. You can find it at Amazon.com by typing in search word "MAM baby bottle."
... I hope this helps. Take care and good luck,