If your muffins are tough or dense, you likely overmixed the batter. You only need to mix the batter until the ingredients are incorporated, this is essential for a fluffy muffin. A couple of streaks of flour are fine when mixing, but if you continue to mix beyond that you end up bursting the air bubbles causing your muffins to be dense. Less is more here! Otherwise, you may have accidentally used too much flour. Be sure to use the scoop and sweep method while measuring flour. See the ingredient info section above for more information.