mirami: The disappearing faces may have something to do with the scale of the model. Small size models with microfacets are a known limitation in Sketchup. The workaround is to scale up by some factor like (10), perform the operations, and then scale down by (0.1). There is a native tool to "repair tiny triangles" in a right click menu after selecting the perimeter edges. If it does not show up, there is also top line menu Tools>Utilities>Create face. This could end up being a lot of "knitting", as I call it. (It may be better for you to direct your questions in the other thread "on designing a shoe".)

mirami I think you would enjoy this thread, and I hope urgen notices your post, as I believe she is the master of this. http://forums.sketchucation.com/viewtopic.php?p=17233#p17233

thanks a lot for your reply. As a matter of fact, I suceeded to solve the problem this morning. Vista had placed one .rb files into material, don't ask me why!!! I took it away and placed it at its right place into plugins, and since everything is working fine!! Many thanks Michel