Flow Along Surface / not complete surface

Hi, I am trying to use Flow Along Surface with this target surface, but it only let me select the blue part, while it’s one surface. (extracted the blue part to show the problem).
Other surfaces went without a problem, any clue?

Thx

Hello - I guess you’ll need to merge the surfaces and then Rebuild, or, better, Rebuild the rounded rectangle curve that made the object with a lot of points - maybe 100, and extrude that as a temporary target surface that is all one evenly parameterized surface.

-Pascal

That work perfectly, thanks!