I regularly sharpen serrated S30V on the white stones. No problems at all.
There is no need to go to the grey unless you have a desire to round off the tips of your serrations.
Granted, I don't know what shape you knife is to start with. If it isn't getting sharp after 20 strokes per side (set to 40 degrees) with the white rods, then you are doing more here than just maintaining the edge.
If you have to do more than 100 strokes per side and the edges still haven't come together, than you would be best served to mail it to Golden and let them reprofile it for you. Sounds like the edge has worn down though use or improper sharpening techniques in the past, and the rods aren't even getting to the actual cutting edge yet.
I salvaged a very beat up endura (had to heavily back bevel it jsut to get to the edge), but I don't think I would want to do it again. Next time I will just mail it in and let them reprofile it for free and save my wrist for playing golf.
It's also possible that the stones have plugged up with metal and they need a good cleaning (bar keeps friend and a sponge with hot water).