Make sure the seatalk is getting power. It may be coming from your autopilot, which probably has an internal 5-amp fuse that powers the seatalk. The red seatalk wire should be 12v+.
Also, you can try disconnecting seatalk instruments on the boat one at a time until it starts working.
Yes, bobwebster is on the right tack! - I have had similar problems with my ST60 - RAYMARINE advised me to check the SEATALK system by changing the backlighting on one connected instrument. If the backlighting doesn't change on ALL connected instruments when you change the one, there is a problem. Disconnect instruments one at a time, repeat the backlight test until you have eliminated the problem instrument (when you disconnect the bad one, changing backlight setting on one of the remaining will change backlight on all other connected instruments). Let us know what you find!