There are many, many reasons this could be.
I would first print your 2016 and 2017 returns and compare them line by line. This will help you determine what has changed, if there are any discrepancies/items you missed or if you need to change any of the information you entered into TurboTax.
There are several factors that go in to determining the taxes you owe.
If your income increased, which might have put you in a higher tax bracket, it could have reduced or eliminated some deductions or credits and the tax laws changing year to year will determine the amount of taxes that need to be paid as well.
Also, you may have paid in a lesser percentage of Federal with holding per dollar than you did last year.
If your income decreased, that sometimes can reduce your refund due to you not having enough earned income to receive the deductions or credits you did before.