burping

My 9 week old is so hard to burp ?. I try burping her throughout her feeding because she suffers from bad gas . Half the time I'm sitting there patting her back for a half and hour and she won't burp . He not burping isn't helping with her gas at all . I try multpule positions every time . What should I do ? Any suggestions on helping her gas pain ? I've tried gas drops and massing nothing has helped . Even changed her foumual three times . Been to the doctors a few times about this and NOTHING ! . She's also been in the hospital for four days prior for not eating . She was going good the first week or two that we were home and now she's gassy again