My baby walks on flat ground but can't walk on grass and sand?

This difficulty on different surfaces is completely normal. The balance, muscle memory, and stepping reflex your baby developed for walking learned first on flat, stable, and familiar surfaces. On grass, sand, rocks, or sloped ground, there's different pressure, slipperiness, and height variation with each step, so the brain struggles to process this new sensory information. This destabilizes their balance and causes them to fall. This is a natural part of development; with practice — that is, walking experience on different surfaces — adaptation speeds up. On average, this skill noticeably improves between 12-18 months of age. Still, to prevent head hitting hard surfaces during falls at the park, I recommend supervised walking and holding their hand for support when first trying on grass and sand. If falling frequency continues beyond two weeks or there's noticeable decrease in walking motivation, consult your pediatrician.