It's probably because it hasn't been kneaded enough or the dough is getting too warm. The final dough should have a silky/shining surface and feel cooler than your body temperature. The texture should be soft and a little bit sticky but the dough shouldn't be hard to handle. If the dough is getting too warm before you're done kneading, put the mixing bowl in the fridge to cool it down for 20 minutes before you knead again.