Flat bottles at two weeks is almost always a conditions problem, not a dosing problem — the sugar calculation being correct doesn't carbonate the beer by itself; the yeast still in suspension has to actually ferment that sugar, and that takes both warmth and time.
Rule out temperature first
Carbonation is a real, if small, fermentation, and yeast activity slows sharply as temperature drops. A batch dosed correctly for 68°F conditioning — say the standard 116.7 g corn sugar for a 5-gallon batch at a 2.4-volume target — but then stored somewhere closer to 60°F takes noticeably longer to carbonate than the same batch held nearer 70°F, simply because the yeast is working slower at the cooler temperature. Check where the bottles actually sit, not where you intended them to sit — a cellar, an unheated closet, or a cool basement corner can easily run 10°F below a "room temperature" assumption.
The usual checklist, in likely order
- Not enough time yet. Two weeks is often enough for a standard-strength ale at a normal conditioning temperature, but a bigger beer, a beer conditioned cool, or a beer using a less vigorous priming sugar can genuinely need three to four weeks or more. Patience resolves more "flat" reports than any troubleshooting step.
- Underdosed priming sugar. If the actual dose came in well under the calculated target — a scale error, sugar spilled during transfer, or a miscalculated conversion between sugar types — the batch simply doesn't have enough fermentable sugar left to reach the target CO2, no matter how long you wait. Double-check what you actually added against what the calculator says you needed, including the conversion if you used table sugar or DME instead of corn sugar — see corn sugar, table sugar and DME priming ratios.
- Weak or old yeast at bottling time. A beer that sat in the fermenter for a long time, or was cold-crashed hard before bottling, can have too little healthy yeast left in suspension to referment the priming sugar promptly. A small, gentle rousing before bottling (never a vigorous shake, which risks oxidation) can help; some brewers add a small measured dose of fresh yeast at bottling specifically for this reason on long-conditioned or high-gravity beers.
- A genuine seal failure. A bottle that never carbonated at all, sitting flat alongside others that did carbonate, often has a bad cap seal rather than a dosing or yeast problem — check that bottle specifically for a hiss on opening (or its absence) and a firm cap seal, rather than assuming the whole batch shares one cause.
Give it warmth and time before anything else
Give it another one to two weeks somewhere warmer if temperature is the likely culprit, and check a bottle again before assuming anything's actually wrong. If it's still flat after a genuinely warm month with no improvement at all, that points to underdosing or dead yeast rather than a patience problem — and if you're at all unsure how much sugar you're dealing with, use too much priming sugar is the opposite failure mode worth reading too, since overcorrecting a flat batch by adding more sugar on top of an already-correct dose is exactly how that mistake happens.