Need to refund voluntary deduction

We have an existing deduction currently set up, non-recurring, voluntary deduction.  It is set up for miscellaneous refunds.  I can put in a negative amount and it will refund that amount as net pay.  We just set up a new deduction, mirrored the setup exactly as the existing miscellaneous deduction element.  However our new voluntary non-recurring deduction only appears to work if there is a gross earnings.  Whereas the existing voluntary non recurring deduction does not need gross earnings to work.  What would cause this to occur when the set up looks exactly the same?