Running March payroll for 2 directors. Both are paid the same / below NI threshold & have same payroll settings, however one has NI calculated, the other not. Why?

They have opening balances (earnings only, no deductions) from another payroll system but within the 20/21 tax year.  

I did get hold of support in the end - they helped me post an adjustment and resubmit the payroll, but couldn't figure out why it had happened in the first place.

Anyway, problem solved for now - thank you for the reply though.