For Next Loop with IF statement inside does not loop

I am working on a project (that should be simple) but I cannot get it to work. All I need it to do is to loop through a range of cells that will always start in cell D2 but will have a different ending row count every week and look for any cell that shows #N/A (from a previous vlookup formula) - then show an input box, we input the correct ID# which gets put into the cell instead, and move onto the next cell until there are no more cells. I have tried various ways of doing this and the inputbox part works (i.e. if I have #N/A in the first cell it completes what I want it to do) but it never moves on to the other cells.... please help! Here is my code at the moment (this is just an excerpt so I copied all my variable declarations even what I don't use)...

Sub Team3Test()

Dim NewSheet As String
Dim c As Range
Dim i As Long
Dim TestRange As Range
Dim cell As Range
Dim b As Range
Dim Employee As String
Dim SSID As Long
Dim NumRows As Integer
Dim myrange As Range


   Set b = Range("D2")
    NumRows = Range("D2", Range("D2").End(xlDown)).Rows.Count
    Set myrange = Range("D2:D" & NumRows)
    For Each b In myrange
            If Application.IsNA(ActiveCell.Value) Then
            Employee = ActiveCell.Offset(0, -3).Value
            SSID = InputBox("Please enter the Staffsuite ID# for " & Employee & " :", "New Employee Found")
            ActiveCell.Value = SSID
            End If
    Next
    
End Sub