This is a very good question. Depending on who set things up have them get in touch with the appraiser. A reputable one should re-evaluate the appraisal based on your questions and have reasons why they came up with their appraisal. If that doesn't work since a low appraisal affects both parties see if you can have another appraisal the lender will agree to and split the price between you and the seller if necessary.
Appraisals are rather tricky right now because prices are inconsistent and many appraisers are having trouble finding comps to the particular property. When I am the seller's agent I have my comps ready and show them to the appraiser. Many times they consider these as part of the process.
I hope this helps! Good Luck!
Keller Williams Realty - Leesburg, VA