It's possible that you have a faulty scart lead, or maybe if it's cheap there is no wiring in it for RGB- only comp. Open the cover off the scart plug and check there are wires going to pins 7,11 and 15. If not, get a new scart lead. I have heard of some early sets that have a single scart socket that does not have the RGB connections but these are very rare. Some older TV's won't sync to this properly and may Roll vertically.
Most modern sets cope without problems.
All diagrams are for the underside of the board.