Snapper Nija self push mower won’t start

[Alfredo_Jimenez]

Hi! This is my first post, I bought a used Lawn Mower, Snapper Ninja Selfpropel, so I am not sure if everything is original, when I picked up it started right after 2 pulls, it worked the first day I used it, then it requires to many pulls, I even have a pain on my shoulder. I removed the carb and it was cleaned and it seems to have new parts, filter looks clean, spark plug looks new. What else could be?