Make sure every repetition is the same!!!
Think about it : quad extensions for example. Often I get clients to do 20 + reps of these and sometimes with isometric holds.
Key here is focus on making every rep the same!! Is your 20th rep the same as your 1st rep? Almost always NO. Before you worry about upping your weight, focus on making every rep as good as the first if not better!!
As you progress through the set obviously your quads (or target muscle is going to fatigue), thus the other surrounding muscles are going to start to take over more. Another good example is a lateral raise.. the first half the set you can generally feel your “side shoulder” and as you work thru the set your front delts, traps, etc start to take over.. And this is normal!!!! However think about it and FIGHT the urge to let those other muscles take over.
Of course this is going to be hard and it will take practice. It’s not going to come in one day. But work on it, think about it, and execute! You WILL see gains.