The first time I thought a cider had stuck, it hadn't. I'd been staring at the airlock, counting bubbles, and when they slowed down on day four I assumed the worst and nearly dumped a perfectly good batch of Nottingham-pitched cider that was, in fact, still working away fine underneath a loose-fitting lid. Since then I've learned to stop watching the airlock and start watching the hydrometer, which is really the only tool in this hobby that tells you the truth.
Stuck versus just quiet
Airlock activity is a proxy, and a bad one. CO2 has to escape somewhere, and if your lid seal is even slightly loose, or you've bumped the fermenter and reseated the lid, gas will leak out around the gasket instead of bubbling through the airlock. I've had ciders that looked completely dead from the outside — no bubbles for two days straight — that were sitting at 1.020 and dropping. The airlock told me nothing useful either way.
The only real signal is gravity. Take a reading, wait two to three days, take another. If the number hasn't moved — not "moved a little," actually flat — across two readings spaced a few days apart, you have a genuinely stalled fermentation. If it's still creeping down, even slowly, leave it alone. A lot of stuck-fermentation panic is really just impatience meeting a strain that ferments unevenly, especially in a cooler room. Give it time before you diagnose a problem that isn't there.
Temperature that's too cold for the strain
This is the most common cause I've run into, and it's usually self-inflicted by putting the fermenter somewhere convenient rather than somewhere warm enough. Most ale and cider yeasts want to sit in the mid-60s to low-70s Fahrenheit to stay active; a garage or an unheated closet in cooler months can easily sit below that. Yeast doesn't die at these temperatures, it just goes dormant or works so slowly that it looks stopped. Check where your strain's ideal range actually falls — it's printed on the packet — and compare that to where the jug has actually been sitting, not where you assumed it was sitting.
Weak or expired yeast at pitch
Yeast that's past its use-by date, or that was stored somewhere hot before you bought it, can have a fraction of the viable cell count you're expecting. It'll often start fermentation just fine — there are usually enough live cells to get things going — and then run out of steam early because the population was too small to finish the job. If you're using packet yeast, check the date before you pitch, and if you rehydrate rather than sprinkling dry, follow the temperature guidance on the packet; rehydrating in water that's too hot or too cold kills a chunk of the cells before they even get started.
Sugar that outran the yeast's tolerance
Juice isn't a fixed product. Sugar content swings with the apple variety, the season, and how the juice was pressed, and unusually sweet juice can push the potential alcohol above what your yeast strain can handle. Every strain has an alcohol tolerance ceiling — a basic ale yeast might tap out around 10-12%, while something like EC-1118 pushes considerably higher. If your starting gravity was unusually high and fermentation quit right around where the math says your strain should max out, that's very likely what happened, not a mechanical problem with the yeast itself.
Missing nutrient — the cider-specific culprit
This one catches people who've brewed beer or wine without trouble and then hit a wall with cider specifically. Yeast needs more than sugar to reproduce and finish fermenting cleanly — it needs nitrogen and a handful of other micronutrients. Grape must and grain wort both carry a reasonable supply of these naturally. Apple juice largely doesn't. That gap is exactly why cider stalls more often than beer or wine made from scratch: you're asking a yeast colony to do a full fermentation on a substrate that's basically sugar water with some acid and flavor compounds, and nothing to actually feed cell growth. A stall that isn't explained by temperature or a high starting gravity is very often this.
The fix, in order
Don't reach for new yeast first — it's the most expensive and most disruptive option, and it's rarely the actual problem. Work through these in sequence:
Warm it up. Move the fermenter somewhere that sits within the yeast strain's proper range and give it a few days. This alone restarts more stuck ciders than anything else on this list, because a cold-stalled colony just needs to wake back up.
Add yeast nutrient. If warming doesn't get gravity moving again after a few more days, dose with a yeast nutrient product per the label, gently rouse the fermenter to knock the yeast back into suspension, and wait. Given that nutrient deficiency is the most cider-specific cause on this list, this step fixes a surprising number of stalls that temperature alone won't.
Re-pitch with a fresh, tougher strain. If gravity still hasn't budged, pitch a fresh, more alcohol-tolerant yeast — something like EC-1118 is a common choice specifically because it shrugs off higher alcohol levels. Rehydrate it properly first, and just as important, acclimate it to the batch's current temperature before adding it rather than dumping room-temperature rehydrated yeast into a cold jug. A sudden temperature shock on pitch can stun or kill the very yeast you're counting on to finish the job.
When to just let it be
If your gravity reading lands close to where the math says a dry cider from that juice should finish, you're not looking at a stuck fermentation at all — you're looking at a finished one. Take a taste. A dry, still cider with no perceptible sweetness left and a stable gravity across a few days is done, not broken, and it's ready to rack, bottle, or drink as-is.