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 and prompt you to adjust your 2017 return.
your 2017 Federal form 1040
1) Lower left side of the screen, click last category "Tax 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 or extra zeros
Below are reasons why your refund may have decreased.