I sell personalized note cards and won't print until the buyer approves the proof first. I would say about 10-15% of the time, I have to pursue for days until I get a response. :(
I am thinking of putting "CARDS WILL NOT BE PRINTED AND SHIPPED UNTIL PROOF IS APPROVED" in the beginning of each listing (I have it in my listings but it might be missed half the time). It can be frustrating and I worry that holding an order up (while I wait for a response) might result in a negative feedback. Sigh.