It is an art. Make sure you press the tape down firmly across the whole strip.

that kinda tape always comes undone for me too, i prefer using this stuff:

House pixies for sure

Oh yeah that is not good.

thaumata:

I was a shipper for years and Scotch heavy duty is your friend. Also, a piece perpendicular to the flap opening provides major strength. It never caught on because of cost but it's truly a game changer.

LOL