One possibility is that you mistakenly mailed your 2017 tax payment using a 1040-ES voucher instead of a 1040-V voucher.
If so, the IRS would have credited your payment as an estimate toward your 2018 taxes instead of applying it to your 2017 tax bill.
You can view your IRS account here: https://www.irs.gov/payments/view-your-tax-account It will show your payment history for up to 24 months.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
