What sort of planform are you using now? And is this a flying wing or conventional design?

Just wondering if you're tapering the wing too much. For models I've usually seen that you don't want the tips to be less than 70% of the root if you want to avoid undue tip stalling.

The other thought is that you may be better off using proper airfoils rather than using that contstant thickness core with the airfoil foreshortened at the tip. It may be too blunt and the air can't follow around the curves?

Could you trace around the templates and scan them in or do something similar so we can see what you're working with?