My key fob not working on my car?

I have a renault clio 1.9dti diesel year 2ooo. I've changed the battery in my fob, the light comes on but the central locking doesn't work, and when i put the key in the ignition, the immoberliser light flashes fast. The only way i can start the engine to start, is i have to push the button on my fob can you help please?