Tell the client that the spotty communication has become an issue, as its preventing you from finishing their work and moving on in a timely. THUS, they have say... two final weeks to give you everything you need from them so you can finish the ref.

If after that two weeks they have not gotten back to you with the appropriate information, you will either A) finish the commission with what little info you were given previously and draw three expressions of YOUR choosing or B) cease work on the ref sheet, give it to them as-is and refund for work not done.