Are you even squeezing?
As you stroll through gyms you've most likely seen trainees with less than desirable exercise form. Whether that be barbells swinging with arched backs while performing curls, bars being bounced off the chest during bench presses, lifting with the back during the deadlift or performing reps at maximal speed.
The lack of proper exercise performance not only can lead to sever injury, but it also leads to lesser than ideal stimulation of the target muscle.
To prevent this from happening and to ensure you're getting full benefit from each exercise make sure you first isolate the target muscle. With the tricep pushdown that's done by keeping your elbows tucked in to your sides and not letting them move throughout the full repetition. It's very common to see the arms swinging in an effort to lift heavier weight. This is counterproductive as you're now bringing other muscles into the equation and therefore taking away from the triceps.
Ensure you're pushing the rope all the way down and to your sides so that you're in full extension which will recruit the most muscle fibres.
And SQUEEZE! Over do it if you have to.
You want to really feel the contraction in the triceps.