I havn't had this problem before, but I've just run into it.
I've created my own segments and exported them, but once in FPSC they act weird in the editor. If I place more than 2 tiles down the previous one vanishes, but when you move the mouse away (The dark area that renders the models on the editor) the segment is still there. When the game is built the segments do not show up at all. I don't know why this has happened, because some of the segments came out fine
EDIT: Oh yeh, i'm getting Sign IV soon aswell so that should help create the segments...
Marc