Self Assessment doesn't add up?

Tax underpayments are often collected with a gap year in the middle.

For example if your 2016/17 underpayment could be collected through your tax code it would done via your 2018/19 tax code, starting next April. The current code for 2017/18 wouldn't be adjusted part way through the year to include a new underpayment.

So if you had an underpayment for 2014/15 it would have normally been identified in the 2015/16 tax year and then collected during 2016/17.

But inclusion in your tax code would normally mean your employer deducted extra tax.

Without more info about your tax history - have you done tax returns for a few years? What tax code was issued by HMRC for 2016/17? How was the tax code made up? What tax code did your employer use for 2016/17?