Does you cable actually fit into both your laptop and TV?
There are different types of S-Video.
You have 4-pin, 7-pin and 9-pin.
Your cable may be a different pin to your Laptop's onboard socket.
Heres what fits into what:
4-pin plugs can go into 4 and 7 pin sockets, but NOT 9.
7-pin plugs can only go into 7 Pin sockets, NOT 4 or 9.
9-pin plugs can go into 7 and 9 pin sockets, NOT 4.