Are you using always the same recipe (it might not be a good one) or does this happen all the time even with other recipes.?
If it;s the same with all recipes, it's probably the temperature or the oven.
If your oven is hotter on top, it will cook the top too much and make it hard.
If the cupcakes are too close to the top it will cook them too much.
If the oven is not calibrated right it might make them cook too hard.
Try another recipe and see. But remember, you have to use a recipe but you also have to use your good sense. If their temperature is not working, maybe it;s too high or too low.
You might put them on a lower rack.