I have tried setting this at multiple surfaces:
- different signals
- different signal counts
- different platforms
- turning the signal on and off
- moving the signal from green to red to both to none and then back again
- renaming the platform to have the same number as the signal count.

I have no idea what I'm doing wrong. I'm happy to provide any info needed but I don't know how to get more

What other mods are you using? (Especially mods that affect the rocket silo)

Do you get a "Zzz" icon on the rocket or does it just not respond at all?
Also, one thing you might miss if you're just testing; it won't launch an empty rocket.