all i can suggest is to go through the usuall stuff
double check all the cables
make sure the game is in right
since your using RF, hook the jag straight to the TV (not through a cable box or a vcr)
make sure the L and H (channel 3 and 4) switch is in one place or another, not in between
shake, blown on and wiggle everything (it's more of a superstition but eh..)
and if all that fails, you probably have a bad RF cable
(note: if you're not actually using an RF cable, then composite video doesn't show up on channel 3 or 4, it's on imput or video 1 or something.)