I had Restylane for under eye circles but Dr said I had thin skin and couldn't inject all of the vial. Is that normal?

Thank you for your question.  Every patient is different, and the exact volume of filler necessary to treat a particular patient depends on a whole host factors, including their specific facial structure and anatomy, the thickness of their skin and the fat the under their skin, and their specific aesthetic goals.  It's impossible to comment on your specific needs without assessing you in-person, or at least reviewing your before an after photos.  That said, there certainly are some patients with mild volume loss and thinner skin who do not require a full syringe of Restylane for optimal correction under the eyes.  If you find you need additional filler in the future, it is much easier to add more filler than it is to remove filler you don't like.  Best of luck!