I have had a recent phone call with Google who told me that my seller rating stars aren't showing because not enough (minimum of 150 in the last 12 mts) are of a high quality. Well this doesn't explain a lot and I checked my reviews and one thing I can see is that our customers are extremely brief and we have many reviews that are very short.
I understand you don't want to discourage customers from leaving reviews, but if Google potentially ignores short reviews then what good are they for me from a marketing perspective.
My idea is to add a dynamic character count to the side of the comment box which simply changes colour. Something like
< 25 characters = red
< 50 characters = yellow
>100 characters = green
And maybe even a little tool tip that explains to the customer that they should include more detail about their feedback which is more helpful to the business to know what was good and bad etc.
This might just help to get customers to write a little more than just "excellent service"