Feeling like I can’t empty my breasts?

Hey ladies, I’m looking for some advice. I have been exclusively pumping for 4 weeks now, which was going great! I was getting around 8oz a session. Over the past few days my right boob has become hard, swollen and sore. I’m still pumping on the same schedule every 4/5 hours but I feel like I can’t empty breasts completely. This morning my right boob was so sore and I had leaked all over my bed despite only pumping 4 hours earlier. Is this a pump issue? Or am I doing something wrong? I use the Momcozy S9 PRO for reference! Really appreciate any advice