Just some tips from an experienced atlatl user. First things first, I
started using dowel darts with duct tape fetchings myself back in 7th
grade. That taught me a lot.
It's all about the darts. Your darts are too thick and stiff. The more
flexible the dart is, the better. Try using a smaller diameter or longer
length. Dowels however usually only come at 48 inches, so aim for a 3/16 or
5/16 diameter. Next, put some front weight on those darts. The nails were
perfect, and your accuracy and efficiency is lessened now that your darts
are almost back heavy.
Next, on the atlatl, good design, but way too long for such short darts.
Average atlatl is 24 inches, the average dart is 72 inches. So shorter
darts, shorter atlatl. Aim for no longer than 18 inches with those short
darts. I think you'll notice an improvement in accuracy.
So flexible, front heavy darts will increase range, power, and accuracy.
And a shorter, more proportional atlatl will improve accuracy.