It takes time for broadcasters to make programmes available on demand after they go to air, particularly live broadcasts (like the news for example).  For programmes that can be made available for catch up on demand, the ingestion process typically takes 4 hours so you may not see it available on demand straight away.

Also good to keep in mind that not all programmes on air will be available On Demand after it airs; while some programmes will be exclusively available On Demand or be available On Demand before it airs – it all depends on whether the broadcaster has the rights to make that content available for streaming or for broadcast, or both.