As we do not have access to your 2017 and 2016 returns we can only offer some possibilities

Try a line by line comparison of your 2017 and 2016 returns-something may jump out at you.

To view your 2017 Federal form 1040
1) Lower left side of the screen, click last category "Tax Tools" .
2) Select "Tools"
3) Select "View Tax Summary" under “Other Helpful Links”
4) Then back to the left-side of the screen, click "Preview 1040"

If a line by line comparison does not reveal the difference, you should revisit all of your entries for accuracy and completeness starting with your W-2(s). Look for misplaced decimals and extra zero's.

Below are reasons why your refund may have decreased.

https://ttlc.intuit.com/replies/3301788