I've salvaged a bow with a similar problem once by adding in the central 60 cm / 2ft a few mm of plant fiber backing (sisal cordage untwisted to raw fibers). Reverse drew the bow before gluing the backing on (hide glue), so to pre-stress the fibers when dry. Backing applied well over the fades, and fades out mid limb. Let it dry for a week, then restarted tillering. If it still isn't drawing enough, repeat. Afterwards, I backed that bow with cherry bark to cover the fibers. In addition, it makes the back really water proof.
Two years and many hundreds of shots later, it's still shooting fine. Other specs: red oak heart wood, 67", last 2" mildly flipped, pyramid front shape from 4.5 cm /1.8" outside fades to 1.3 cm / 1/2" at tips. pretty even thickness taper, c. 1.4-1.5 cm thick. Slightly toasted on the belly, working fades and grip. Draws about 45# at 27" but can be drawn no problem to 29", has c. 1" resting set, 1.5" after shooting, shoots c. 155 fps at 10 gpp.
Joachim