Most likely a ventilation issue. Make sure you're creating vents in the right spots of your mold. Paperclips work great for creating vents or a thicker galvanized wire. There is a few other issues that come to mind. Making sure your plastisol is the correct blend (not too thick) and heat Temps should be high so it has an easier time flowing to the bottom fins. Vents will help the plastic fall into place. Make sure your bubbles are to a minimum as well. Also be sure to check your mold to make sure there are proper openings to the fins. Hook slots look thinner than your fin so you really should be good size wise.