Could flipping your bottles upside down every day actually fix a lack of carbonation?
Earlier in my brewing history, I made a really nice Belgian Tripel. It had great flavor and aroma, but came out of the bottle totally flat - not a hint of carbonation. I tried giving it more time, but that didn't work. I tried moving it outside where temperatures were in the lower 80's. No good. After two months, the beer remained flat.
I reached out to a home brewing magazine for advice. They suggested opening some of the bottles and pitching in some dry champagne yeast, then shaking the bottle gently to rehydrate the yeast. This, combined with another month, did not solve the problem.
Eventually, I poured the flat beer into a uKeg stainless steel growler and force-carbonated it in there.
After that, I tried to live by a simple rule. If the beer I brewed was at or below 8% ABV, I would condition it as-is. If it was over 8%, I would pitch in some fresh yeast with the priming sugar. Typically this would be champagne yeast (since it's cheap and generates lots of CO2) or CBC-1 bottle conditioning yeast. This seemed to solve the problem.
Fast-forward to today. I had a beer brewed over a month ago which, despite four weeks in a 76-78F environment, was more or less totally flat. Open a bottle and there is no hint of a hiss.
I started doing searches and found a suggestion so simple that I wouldn't have believed it could work. It was a no-risk suggestion, so I decided to try it.
What was it? Every few days, turn all the bottles upside down (or if they're upside down, right-side-up). This forces the yeast to fall through the beer, which is primed with sugar. Some of the yeast will in theory be roused into life and do some carbonating. Each time you flip the bottle, there should be more active yeast and more carbonating taking place.
For the past seven days, each night I have visited my bottles. The 12-ounce bottles are inverted in their six-pack containers. The 22-ounce bombers are picked up, inverted in my hand, shaken very gently, flipped back upright, and returned to the conditioning box. (This is an insulated cooler with a heating element and controller that keeps the inside at 76F or a yeast-compatible temperature.)
Last night, I took a 12-ounce bottle and refrigerated it. Popping the cap, I was immediately greeted with the hiss signifying the release of carbon dioxide. It had carbonated at last!
If you are encountering an issue where your latest home brew hasn't carbonated properly, I would definitely recommend trying this technique. It seemed to save the day for me.
Update 10/08/2017: Recently I had another batch fall victim to undercarbonation. When a bottle of the beer (11.4% ABV or better) is opened, you get the familiar hiss of carbonation. When the beer is poured, however, it's totally flat. These bottles were kept in a 76F container and inverted daily for about three weeks, without any carbonation increase. I then increased the temp to 80F and inverted them daily for another three weeks with no benefit. So clearly this tip does not work in all cases. For information on what I did next, see my post on Rescuing Undercarbonated Beer.