Convex Hull problems

[Stiglr]

Why is it that sometimes, when I try to create a convex hull from a pair of cross sections, the software often just 'decides' which vertices to ignore completely in making a new shape?

For example, say I'm modeling a vertical stabilizer and rudder. I make a poly shape of the filet of the piece and another of the profile of the piece and arrange them perpendicular to each other in the proper place to describe the entire tail unit. Now let's say the trailing edge of the rudder has a nice curved edge leading towards where it will join the fuselage...

I create the polys of both shapes, select all the vertices, and do a Convex Hull. The result I get is a straight line between the trailing edge, and the polys I'd set to describe the scalloped edge just aren't there anymore. The rest of the shape is exactly what I had in mind.

What's the trick to getting this feature to obey instructions?