Without looking at your return it is impossible to know why.
These are the most common things that effect your refund or balance due:
- Change in Filing Status;
- Change in dependents claimed;
- No longer have a child, under age 17, to claim the Child Tax Credit
- The amount of federal and/or state withholding;
- Income based credits, such as the Earned Income credit;
- Tax bracket changes
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For the 2018 tax year, a great deal of the tax rules were rewritten. No more exemptions, different standard deductions, etc. Your return in 2018 will look and probably be quite different from 2017 tax year.