You need to know what RPM your'e getting out of that engine at wide open throttle.
It may be possible that if your not getting a decent amount of RPM that the engine isnt developing enough power to get on the plane,
If you have 500 RPM or more available its possible that you could use a slower prop, ie a finer pitch, which would allow the engine to rev a bit more, enough to develop the extra power needed to get you on the plane.
It may be a bit much to expect though for that size engine.
years ago i can remember a CJR14 planed with an old 2stroke 15, but that was marginal. but the engine was so old it was flywheel rated, so probably only 12HP at the prop.
All you can do really is find the pitch of the prop which should be stamped on it and go for a finer one.
Or buy a ratty old 30HP 2 smoke and keep the 9.9 as reliable backup