Are you doing the correct weights and the correct amount of reps ?

Don't stop when it gets hard, push right past that point where you think you should stop until you basically can't do any more! I recommend 12 reps of a weight you find yourself slightly struggling on (where it starts getting sore around rep 8) then take a 45 second break and do another set of 12. Bump the weight when it starts to get easy.

Then move onto another machine.