Strange falure in Solid subtraction

Attached is a drawing that I'm having trouble with. The gold solid is was created and the cylinder, cone and slot near the x-axis were cut out using solid subtraction.

Then I created a skin from the green curves below the red solid. using extrude surface along zAxis I created three red solids I then used Solid subtract to create a
channel between the cone and the slot the two solids that were created from the curved sides of the skin surface subtracted correctly but the main section failed in a
funny way. if you hide the red solid it looks like the subtraction removed the interior of the gold solid but left the top surface. any ideas about what I did wrong?

I didn't realize I could not attach a bbcd file :( so attached is a .dwg the red piece is the curved object protruding from the top of the part.

Please attach a .bbcd file.To do this,,,r click your file and select send to compressed folder.That will give you a .zip file that you can upload here.

Here is the flute-2.zip file an the flute-3.zip that worked.
Additional information the original red object was made from a skin. The rails of the skin were two fit splines. I redid the entire model and used interpolated splines for the rails.
This time the subtraction went fine. Any idea why one worked and the other didn't.

This solid is no good ? Do you see the gaps ?

Also you have an extra surface sandwiched among the 2 solids.

The gaps showed up when I did the first subtraction. They correspond to the two outer most sections of the solid I made by extruding the surface that is left between the two solids.
It acts like the only real solids are the two that are the thin slices immediately next to the rails in the skin creation. The red solid is the second attempt to do the subtract but it does nothing
When I try to subtract it.

Is there a way to test whether a solid is completely sealed? I suspect the skin extrusion left some gaps somewhere but I can't see any..

I've found that extruding one surface of the skin at a time is more reliable than trying to do the entire skin at once. I've also found using the interpolated splines works more reliably than the
splines I create using the fit method, But I've no clue why that is.

In Any case thanks for taking the time to look at this.

Another quick question the flute model has a cylinder and a cone shape on the bottom of the model. Is there a way to create those tool paths in the same file as the top tool paths?
I created a second file, rotated the part around the X-axis and created the bottom tool paths in that file. It works but is somewhat cumbersome.