When you slow cook beef the meat can be tough for a few reasons. If it hasn't been cooked for long enough the meat will still look plump, but it will have a good amount of resistance when you test it due to the fat not having enough time to break down. Leave it to cook a bit longer. Secondly, the meat may have been over-cooked, it will looker a paler grey and have reduced in size due to the lack of moisture. This is much more common in leaner cuts of meat. Generally, the fattier, marbled cuts are much better for slow cooking and that's why I recommend Bavette or skirt for this recipe.