Vegetables cook slower than meat and poultry in a slow cooker so if using them, put vegetables in first, at the bottom and around the sides of the utensil. Then add meat and cover the food with liquid such as broth, water or barbecue sauce. Keep the lid in place. Remove it only to stir the food or check for doneness. Most cookers have two or more settings.
Foods take different times to cook depending upon the setting used.
Certainly, foods will cook faster on high than on low. If possible, turn the cooker on the highest setting for the first hour of cooking time and then to low or the setting called for in your recipe. However, it is safe to cook foods on low the entire time. While food is cooking and once it's done, food will stay safe as long as the cooker is operating. If you are not at home during the entire cooking process and the power goes out, throw away the food even if it looks done.