If the mug cake has turned out chewy or rubbery, it's often because the cake is over-cooked. This can happen because different microwaves can vary in strength, so they may heat differently. If you're not sure, you can always cook the mug cake for the minimum amount of time, and add more as-needed. This can also happen if the ratio of ingredients in the recipe is off. There could be too much oil, flour, or the batter is over-mixed in general.