I'm going to jump in here with a guess as much to test my own beginner's knowledge as to offer a reliable answer. I'll warn you now...there's probably at LEAST a 50-50 shot what I'm about to tell you is complete bunk, so take it for what it's worth.

The reading I've done, and my understanding of props would suggest to me that your prop is just on the ragged edge of providing enough thrust for climb. Most of what I've read suggests that 10 x to 12 x 6 is about right for a .40 sized engine on that plane. Tower, in fact, suggests an 11x5 for that range of engine.

I suspect, simply, that the engine isn't capable of turning such a large, high pitched prop fast enough to generate the speed required to get off the ground. While it would likely FLY the plane, it can't pull it off the ground into flight.

As mentioned above, your mileage may vary.

HTH