A dense pumpkin loaf can be the result of a few things. Using expired leavening agents, like baking soda, and baking powder. If you used Dutch processed cocoa powder, it would keep the buttermilk and leavening ingredients from activating. Additionally, overmixing the batter will make it tough. The batter becomes tough from the gluten in the flour, overmixing activates gluten. Finally, check that you're using pure pumpkin puree and not pumpkin pie filling, which is a liquid with added ingredients.