Why is my beef joint chewy?

Online Answer
Leaner roasts such as rump, chuck, or brisket have less fat marbling, so they can become chewy if not cooked correctly. It is the fat and connective tissue in the roast that is softened by long, slow cooking to produce flavor and tenderness.
Related Questions 📌
If you've ever cooked a chicken breast and had it turn out tough and chewy, it might not have been your fault. ... Woody breast is believed to affect 5% to 10% of the chicken breasts on the market, and though not a health threat to humans, it's certainly not pleasant to eat..
Overcooking can make your meat dry but undercooked meat can be quite chewy. Don't be afraid of an instant-read meat thermometer and pull your meat when it's ready. For naturally tender cuts like beef tenderloin, that can be as rare as 125ºF, whereas tougher cuts like brisket should be cooked to 195ºF.
Good to Know: Connective Tissue Makes Tender Cuts Chewy For example, the nerve end of the strip loin, located at the rear of the muscle from which New York strip steaks are cut, can sometimes have a very thick ribbon of connective tissue running into the meat, which is extremely chewy to eat.
Often, homemade carne asada results in steak that tends to be chewy and rather tough, despite slicing against the grain..
I would say the lamb must be undercooked if it is chewy. What cut did you use? I would try slow cooking in the oven for a decent length of time - say an hour. The trick with the tougher cuts is to cook them slowly, sometimes too fast a boil can toughen meat.