Why do I feel like pooping but nothing comes out?

It can be because of lack of fibre in diet and rectal tenesmus due to intestinal conditions like irritable bowel syndrome, IBD, Hemorrhoids etc. Try to include high fiber foods such as Pears, strawberries, apple, oat meal, brown rice, banana, broccoli, green leafy vegetables (kale, spinach), avocado, carrots, tomato, beans and sweet potato in diet and take plenty of fluids.If issue still persist then consult a doctor.