When you mincemeat, one of the key things that helps mincing machines work well is the amount of fat in the meat. When you cook it, the meat tends to lose fat and becomes more difficult to grind as it becomes tougher.  Another important point to keep in mind is that meat grinders are intended for use with raw meat. More than anything for the issue of health, since cooked meat is easily contaminated and if your grinder is not perfectly clean, it can contaminate the meat.  It should be noted that raw meat can also be contaminated, but later when it goes through the cooking process, these bacteria normally disappear. In any case, we always recommend you to have your meat grinder completely clean.