Appraisals are not divinely inspired documents, they are opinions of value. They are more detailed that a typical agents Comparative Market Analysis but not necessarily more accurate. Two different appraisers can have two different opinions, but when it comes down to it the lender has to feel like there is enough value for them to make the loan.
In your situation we don't know if the 2010 appraisal was high, the new one is low or if both are accurate. Your lender can request an appraisal review, you can have the agent go over the comparables and see if the appraiser used the best comparables available or if they can suggest better ones.
I hope everything works out for you.