Some babies just aren't ready to suck on bottle nipples, and I have seen it in
some babies. They are uncoordinated with  the constant flow and it seems to
overwhelm them. If the mother is willing, feedings go much smoother at the
real thing.
I have seen it with both newborns and with babies who have been in NICU's, who
have been having difficulties with nippling.  Once they get some feedings
established at the breast(sometimes they only need a few) their oral feeds go
much better (of course I try to keep any and all feeds at the breast, but mom
is not always there).
Ah, the wonders of nature!
Fritzi Drosten