you ran the first pay/s for the month with the PAYE period set to Monthly. This will have fixed the PAYE period for reporting for the month, or
on the 2nd or 3rd pay after finalising the first monthly pay, you changed the PAYE period (in Tools > Options > General tab) to Half Monthly. This change will not take effect until your first pay in the new PAYE period.
If you really need to have your pays operating as half-monthly, complete the following:
Restore your backup for the first payroll in that month.
Change the PAYE period to half-monthly.
Finalise that first pay.
Re-enter any pays following that first pay and finalise each one.
You will find that the pays are all now in the first period, in other words the 1st to the 15th of the month. You will be able to check that they are correct by running the Pay Audit Trail report.