When I lay splines on a section of my route they are invisible.I then go back to that area and they are showing again however when I try and drag them into position they vanish again.I'm talking about road lines and markings as well as pavements.It is only happening on this particular section which is still mainly blank baseboard.It is only happening in the one area and I don't understand it.I've tried deleting some buildings nearby but the problem won't go away.Has never happened before.....i'm baffled....HELP .