Guaranteed burping techniques?

My LO is having difficulty bringing up wind tonight. He has acid reflux so we're keeping him upright after feeds and it's actually been getting loads better but tonight he just can't bring anything up. I've tried him across the knees, on the shoulder, and sitting up, and have tried sit ups, knees up, and massage, but nothing. I can hear it moving around in his little gut and belly so there's definitely something in there. Any fool proof ways of getting up wind that I'm missing?