HedgeHog, now it happened to me too! What I did was to remove the station from the stop list and add it again. The line was then assigned to another platform. I assume that when you add a station, some algorithm decides which platform works best given the set of all the other lines going through that station. Make sure that all platforms are reachable. The colored line shown when you place stops will indicate exactly which platform the line will be using. Give it a try, see if it helps!