I ran into the same problem.

Scrub the hull and deck with a scothbright pad and soap and water. the blush and oils from your hands are causing the tape not to hold. The strapping tape will now hold very well.

I spread my hull back apart with some boards just below the joining area which I pulled out after it was together. You can even saw them out if you have to. You can also place in a few forms that you cut off some of the top area under the deck.

Recruit a few helpers. A few sets of hands helps a lot.