The 330 Ohm resistor is correct for a CSYNC cable on unmodded and properly modded consoles. It works together with the 75 Ohm termination in the OSSC to reduce the voltage.
The only possibilities are your SNES not outputting CSYNC (unlikely unless its a 1-CHIP-03), defective RGB cable or defective OSSC. You can rule out the OSSC by trying another console on the SCART input.
Another possibility is that you changed H-PLL Pre-Coast and Post-Coast settings but even then you should be getting occasional sync.