Stuck fermentation
Term from the Brauwerkstatt brewing glossary, category “Faults and hygiene”.
Explanation
Fermentation stops before the expected final gravity is reached. Common causes: too cold, too little or tired yeast, too little oxygen at the start or a hot mash that created a lot of unfermentable sugar. Moving the fermenter somewhere a little warmer and gently swirling it often helps; otherwise add fresh yeast.