Route maps on walk pages were sometimes failing to draw on slow connections, and in the app when installed on a phone. The request that fetches the route gave up after a few seconds, and once it gave up the map showed no route at all. It now waits up to 12 seconds, which is long enough for a slow mobile connection to answer.
See also: Planning and following walk routes in the how-to guides, which walks through drawing a route, generating and editing its directions, importing from OS Maps and following a walk on a phone.