Why is my lifetime learning credit coming up as 10% instead of 20%?

I filled in the amounts from the form 1098-T that the school sent me, but it did not include the tuition I paid on 12-28-2017 for the quarter beginning in January 2018.  The school said that they could not issue a revised or additional 1098-T.  I inserted the December payment as additional payment not listed on 1098-T.  My total education expenses paid in 2017 was $7089 and Turbo tax is telling me that my credit is $710. At 20% it should be over $1400.  My income for 2017 was less than $20,000.   Please help.