Stefan....
Try reducing your control movements.
All you should need is 10 degrees for the elevator and even less than that for the ailerons.
Everything else sounds OK, but you do not have a large amount of power at your disposal.
If the CG is on the verge of being too sensitive, if the power is marginal, if the controls are set too be too twitchy...then this combination of conditions will make trying to fly beyond 15 feet difficult.
What can sometimes happen is the engine is set for almost max rpm on the ground because it has disappointing output. Then the hand launch is a strong throw that causes the fuel in the fuel line to starve the engine momentarily and at the most critical time the power drops below a usable level.
Under 12 ozs is an excellent weight for that size of plane..! give the model a level push away from you at eye level. This is all it takes if all the other conditions are met. Allow the plane to gradually swoop down as it gains speed and it will be flying on it's own right at the 15 foot mark, ready to start it's climb to freedom.
Measure all of your trim settings on a flat table with the model set perfectly level. Measure the same distance off the surface of the table to all parts of the horizontal stab / elevator, center of leading edge of wing and center of trailing edge. If the model is perfectly trimmed it will not need much assistance to fly. Make sure that you do not have any left thrust on the engine..a small amount of right thrust is better if you have made a mistake....but perfectly straight forward thrust angle is what I aim for with a model like this.
To check thrust angle, remove the prop and bolt the model / engine to a perfectly flat plate and allow the model to rest on it's nose.
Now take a carpenter's square and use your eyes to judge how squarely the model stands while looking at it from every possible direction. You can also take "key" measurements at this time off the table to each wing tip.
I use 1/8" round bamboo sticks for the wing struts on planes this size and push them through tight fitting holes in the wings...but do not glue them until I get the model all checked out with these careful measurements, then I use thin CA glue to lock the struts to the wings and trim the extra bamboo away with a razor saw.