in Tracy's first book, she says that babies are so relaxed when they take their DFs that sometimes they don't gulp any air down at all and that if u try for a couple of minutes to burp him and still can't, odds are that there isn't any air going to come out, other than that u should burp ur baby as often as u can, have u tried patting firmly on the back (firmly, like u would do the patting in pat/shsh), rubbing in a circular movement on the left side....and i read this in a parenting magazine; turning ur baby's back to face u and gently cycling his legs...when he stops to burp, does he immediately take the bottle again? if so that's probably for the best, that he burps himself... if he doesn't and seems uninterested after he has burped, i guess u can take out the bottle and try to burp him and then try again, so that he will continue his feed...
HTH
love