Hi Psygorn, if you select your surface and turn on control points, you should see that the "underlying surface" of your flat surface looks like this:



It's a rectangular plane with trim curves on it - that's the kind of surface that will be generated by the Construct > Planar command.

For use with Flow in your case here you need to construct a flat surface where the "underlying surface" has the trapezoidal shape to it, not just the trim curves having a trapezoidal shape. There are a few different ways to do that, one would be to loft between 2 lines like this:



If you then turn on surface control points for that shape, you can see that the actual surface has that trapezoidal shape to it, it's not just the trim curves that have that shape:


If you make the base surface like that, and also move it right on the bottom of the shape to deform, you can then run the Flow command on it to get this result (you can hide the object to be deformed after launching the Flow command to make it easier to pick on the base surface):




Let me know if you're still not able to produce this result.

- Michael