I have had the same problem with my Starkites. You leave the plastic stick in and it deforms the flapper in the valve. Mine got better after inflating them while it was warm and letting the air pressure push back against the flap while the rubber is warmer. I would say above 80 degrees. Sounds crazy but it worked for me. Good luck. I understand how frusterating this is.
dude! you're a freaking genius, I applied some very light heat with a heat gun and a funnel, pumped some air in, and voila! finxed! Holding so far.
Tnx for the observation.