Yes, it is very low.
I have one client who orders about 3 times per week but NEVER writes a review. That is about 150 orders per year that aren't rated - just by 1 client. He doesn't even say anything at all really, just sends the file and accepts my offers, that's it...
But he isn't the only one. Most of my repeat buyers don't rate anymore.
But I guess I have been doing good because my visibility is pretty good I think.