If you had more than one employer and the total of box 4 (only box 4 not box 6) on all your 2016 W2s for Social Security is more than $7,347 you get the excess back on your tax return. And it is for each spouse separately, not combined. Check 1040 line 71 for it. If only one employer took out more than $7,347 you have to get the difference back from that employer and get a corrected W2 form. See IRS publication 17 page 252
http://www.irs.gov/pub/irs-pdf/p17.pdf So check boxes 3 & 4 on your W2. Box 4 should be 6.2% of box 3 up to a max of $7,347. So take the amount in box 3 and multiply it by .062. If box 4 is more than that you need to get a corrected W2 from your employer. But also go back and double check your W2 entries, you may have entered box 3 & 4 wrong. A lot of people who have asked this found they did enter an amount wrong.
A sure way to fix this is to delete the W2 and re-enter it. That may clear something out.