Re: Tubing Trouble
When my son graduated from the kind of tube you sit in to the flat kind with no hole we had the exact same problem. I tried all sorts of things like changing inflation, rope attaching point on the boat, etc., etc. to no real effect. We were exasperated to the point of preparing to return it. As it turned out, as others have said, I was launching too slowly. The hole kind naturally floated nose high when our kid was sitting inside leaning back. You could do a very leisurely start and it would just rise up as you increased speed.
Not so for the one we have now. You've got to let the line tighten at idle, make sure the kiddies are holding on and slid back to get the weight off the leading edge, then hammer the throttle to the stops. The tube pops up smartly and planes out, then I can throttle back while they get settled (and I prepare do dump them azz over teakettle back in the drink...

.)
FWIW, this is the type we have....