Trihorus

Hi, I'm trying the 100 push ups challenge and I'm stuck on Week 5, Day 3 with 120 total push-ups. I've tried this day thrice and failed. I fail at the final 30 push-ups, I can comfortably do about 20 push-ups at once but can't do 30. Is it normal to be stuck for so long on one level? How do I develop that strength? Just liked to share, if you guys have your experience or something else to share, it'll be great!