I personally don't like skull crushers i don't feel my lower triceps working at all
Maybe it's because you were moving your elbows a lot whereas the elbows need to be stationary & keep your upper arms tight like you're squeezing your chest together. Skull crusher makes my arms so exhausted that I just wanna let that weight go & smash my face. It gives me a good pump to my triceps except I don't like it as mush I like dips & kickbacks.