Hi Tom - Flow works just on an individual surface, when you use Join it glues together surfaces into a connected assembly but you still have many different surfaces making that object up instead of just one surface and Flow will only be able to target one surface out of that joined assembly.

In order to do a larger flow you've got to make the initially constructed surfaces to be larger, that usually means making a longer curve to start with that is not made up of smaller segments and is also smooth, things will naturally separate into different surfaces at sharp corner junctures like you have in that shape there.


Maybe try using something like in the attached 3DM file - there I filleted the sharp corners of your original curve and then used Rebuild to make one single segment out of the multi-segmented curve, which then results in one single surface as well.

- Michael