Why can't I crawl?

This morning we wandered in to town, and Orna had an excellent nap.

She's been a bit whiny this afternoon, which seems to be frustration that she can't crawl yet. She can get up on her hands, tuck her knees under but then is stuck (or splats forwards after pushing with her legs) and whinges until she either gives in and flattens down, or is rescued.