When your beef doesn't want to shred, and is tough and chewy, it's because it needs to cook a bit longer. All slow cookers cook a bit differently, so yours may just need a little extra time. It could also come down to the type of beef you used. Using a lean cut won't yield the tender results that a chuck roast will. For the best results, use the cut of meat called for in the recipe, cook as directed, then test to see if the beef with shred with a fork. If it will, it's done. If it doesn't, it needs to cook more.