Meatballs can become tough if not enough liquid was used, if the meatball mixture was overworked, or if the meatballs were rolled too tightly. Make sure your meatball mixture is properly wet. It should stick together with little effort. If not, add a bit of liquid. Make sure to fold the meatball mixture together gently so as not to overwork the meat. Finally, gently roll the meatballs into balls. Don't over-compact them!