Why it happens
Stewing cuts such as beef shin, chuck and lamb shoulder contain a lot of collagen, the connective tissue that makes meat tough. Long, slow cooking in liquid breaks collagen down into gelatine, which makes the meat tender and gives the sauce body. If the stew hasn't cooked long enough, the collagen is still intact and the meat is tough. If it boils hard, the muscle fibres tighten and squeeze out moisture, making it stringy. Lean cuts with little collagen tend to go dry in a stew however long they cook.