The Deltie airshow didn't get up high at all, and I was using the recommended motor. I'm using a very new rod, so I doubt it was that.
With the Elcie G show, I had a weaker motor than needed, and I knew it, but the A10-3T was impossible to find around me at the time. (still hard to get right now)
I am more concerned that I made the booster right. I thought I did myself a favor by using titebond instead of glue, but that may have been a mistake.
I have three bulk kits in front of me, so I have three boosters. I thought I would try using the extra fin that comes with each glider instead of the included fins, since the included ones seem less stable to me, as they are so long in width, but short on the glued end. So I glued up a set of each fins on two tubes and now I think - duh - that's why the fin is shaped that way - the gliders have to sit farther apart, and the extra fins would probably only work on a larger tube.
In any case, I will have a couple versions of a booster, with the right size engines this time, so hopefully I can make it work. I have time for a test run before the event I am planning.