Hi there,
Our DD's had the same issue at times where it could take 20 to 30minutes to get a burp out of them
Although they both used to pass wind a lot and I say if gas is coming out either end then that must be good.
We could always tell by feeling our DD tummy's and seeing how hard it was and that was a good judging point that she needed to burp or she definately would get wind pain.
I found a combination of rubbing for 5 secs then pat for 5 sec seems to work well, give it a try and see how you go?
Good luck
Josh