A lot of times you get stuck in ruts and don't know exactly when to push yourself and increase weight.
Aaron Marino of alpha m. says that if you can do 12 reps with good form, it's time to increase to next increment or level. If you go to that next level and can only do 3, then revert back.
Listen to your body. If it becomes routine, your body is going to adapt and stop changing. You need to push it in a logical and safe way. If you can get 12 reps, time to check yourself and move to next level.