If you're having trouble establishing a mind-muscle connection with your triceps then begin the workout with something that puts the muscle into a shortened position, and work isometrics (holding the contraction) to help with that. Try a pushdown variation. My favorite for this is the reverse-grip pushdown.
Make a concerted effort to roll your knuckles back towards your body and hold that for a 2-3 count for 3-4 sets.
Those guys who think planks and Bosu Ball triceps kickbacks are giving their core a good workout are poor, pathetic slobs. If you really want to work the core, you've got to raise the weight over your head.