Do you have another display like a CRT (without Retrotink) to try, or an alternate method of getting the S-video to HDMI like the converters nmalinoski mentioned?
Sounds like you might be having a similar issue to what I have with my Sega 32X.
Basically, the Retrotink can’t handle colour signals over S-video that are too far out of spec. Other equipment such as the cheapo amazon converters grant a lot more leeway and would help in diagnosing the issue (they should work), but of course they are laggy as hell and stretch the image to 16:9, which is obviously why we want to use Retrotink instead. 🙂