Headcount not increasing when suspending assignment

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Hello, this seems pretty basic, but isn't working like required.

I have an employee that went on maternity leave.  I put her assignment in a status of "Suspended - No Payroll".  However, I am unable to add the assignment to a part time worker.  I get 

You can't select this position because it doesn't have enough vacant FTE nor an open headcount. (PER-1532359)

When I look at the employee on maternity leave, I see suspended for her assignment status.

When I view the position, she shows up still in the hierarchy.

What did I miss??