try to  merge the red surface with yellow if you can..THE YELLOW DISAPPEAR !!

I was looking at your model some more and I agree with Robert above, Surface 1 (yellow) looks like it has some issues as well as some possible self intersecting geometry. I would try to re import it or get a new export of the surface.

That being said, I have seen surfaces that have problems such as self intersection geometry disappear like what you are seeing, it doesnt mean the surface is not there, it only means that it cannot be drawn in the screen. Let me know if you need more clarification. your surface is there, you just cant see it.

Chris

as with any of the issues with imported stuff, you have to do some of the work in surfaces, then turn it back into a solid, then delete a face and work again in surfaces, then go back to solid and back anc fourth again and again and again...it all depends on the file and the types of surfaces, trial and error!!!

what works on the one before may not work on the one you are working on now, sometimes you may have to knit other surface together before the one that is give the issue, instead of 1-2-3-4-5-6 you need to knit 6-1-2-5-3-4 and it works or knit 6-3-1 and knit 2-4-5 and the knit the results of the 2 together and it works.

It's a crap shoot, without having the whole part and knowing what you are trying to do, it's a guess on my part, but i do this alot.

also try to add more sufaces to the knit and it may reapear, has happened to me before.

Message was edited by: Lenny Bucholz been playing, saved as iges, parasolids and brought back into SW .....crapy surface can't untrim but it will surface fill.