The bottle you had last night was perfectly carbonated, the one you just opened was totally flat, and the one you’re about to open will gush all over the floor. When this happens, it’s time to take a closer look at your priming method.
The most common way to carbonate homebrew in bottles is to “prime” each bottle with a small dose of sugar. Yeast consumes this sugar and releases carbon dioxide, which, since the bottle is sealed, dissolves into the beer. The most popular way to introduce that sugar into a batch of homebrewed beer is to mix up a solution of dextrose (also called corn sugar) and water and add it to the bottling bucket as you rack the beer from secondary. If this is the method you follow, then there are a few steps of the process to examine.
Headspace
This is the least likely culprit but an easy one to fix if it happens to afflict you. The more headspace there is in a bottle, relative to the total volume, the more carbonated that bottle will become. That’s why it’s important to push the bottling wand all the way down and allow beer to come all the way up to the lip of the bottle. When you remove the wand, the perfect amount of headspace will remain for proper carbonation.
Bottle Sizes
This is related to the headspace issue because a bottling wand’s geometry is optimized for standard beer bottles and does a fine job with longnecks and bombers. If you have oddly-shaped bottles in your collection, or if you frequently package your homebrew in growler-sized vessels, you might have an issue. Try to stick with roughly the same size bottles from batch to batch.
Poorly Mixed Priming Solution
This is probably the number one cause of carbonation inconsistency. We’re told to siphon slowly and to gently rack fermented beer onto the priming solution in the bottling bucket. The problem with this advice is that turbulence is your friend when you want to mix two different liquids together. It’s why we stir milk into our coffee. If you simply pour coffee onto your milk, it may or may not thoroughly mix. I don’t advocate lots of splashing, but do try gently stirring your beer and sugar mixture with the siphon tubing used to rack it. And then give it five or ten minutes to reach an equilibrium before you begin filling bottles.
If you’re still having problems after addressing these issues, then consider trying some of the new “single-serve” sugar doses available at some homebrew retailers. These look like little pills or sugar nuggets, and you add one or two to each bottle. If you regularly encounter carbonation issues, these may be worth a try, at least as one more data point in diagnosing the problem.