My past experience with Monokote/UltraKote this kind of tip, should have been done with one piece for the whole bottom of the wing and one whole piece for the whole top of the wing. Starting from the center of the wing. Trying to cover just the tip is extremely difficult, as you are experiencing. Do the bottom of the wing first. Iron down the covering at the center of the wing. Do not attach the leading edge of the wing or the trailing edge of the covering. With a heat glove and gun, pull the whole sheet of covering over the center of the tip. As it attaches, continue to pull the covering working forward and towards the trailing edge of the tip. Once secure, then heat and pull the leading edge of the wing covering until secure. Repeat for the trailing edge of the wing. The one mistake I see, is that you are trying to have very little edge on the blue part of the main wing. There just isn't enough material to stretch. Has to be a whole wing panel. Thus, as you pull it, the heat pulls off of the blue too. Repeat for the top of the wing. You are trying to do it with too small of a piece. Has to be one whole panel.