Do you have a loop at some point off-screen? Without any dividing chain signals? If so, it's probably one big block of rail that the incoming train is on, so it's effectively occupying the very rail it's hoping to drive onto. If you put up a zoomed-out screenshot with the rest of the rail on it, things might be clearer.
Add more signals in the rest of the rail track. You probably have one huge block of track which goes from the right exit all the way around the loop into the left entrance. You need to add signals to split that block into smaller bits.
That's probably something tutorials should talk about. Signals are not just for intersections, you need to periodically place them in all parts of the rail.