[dice]

[small bed with wilt]

Sounds like either something is inhibiting rooting there (?)
or it drains incredibly well. Sand deposit under it?

Wi-sunflower mentioned a product called "Roots" that
stimulates root growth. She uses it on seedlings at
potting up and transplant time. Probably any commercial,
water soluble root stimulant would work the same way,
and a home made equivalent is willow branches cut up and
soaked in water. (You could try simply stimulating the plants
in that bed to grow more roots.)

If the problem is fast drainage, you could try a thicker
mulch on that bed.