Why Bluepirnt flowing arrow on playing status doesn't show up?

I am now learning UE5.Why sometimes playing the blueprint,this arrows ,which be shown in the red circle ,are flowing.But in other blueprints,cannot show these flowing arrows

The following case show the flowing arrows

The folloing case doesn’t show the flowing arrows,but can work.

I think it is important for me to debug.I think somebody maybe ask this question,but I dunno know the keyword so serach no result.