VBA merge cells and Bold text of second cell

I am coding a VBA function to merge two cells and then highlight the text of cell 2 with bold formatting

The merging goes well
The call to Sub goes well
But the text format is not applied

I believe it might be caused by the sub executing before the cell is populated with the string - but that's pure guessing - this is my first VBA script

Function boldIt(navn As String, ekstra As String)

Dim ln1 As Integer
Dim ln2 As Integer
Dim st1 As String

ln1 = Len(navn)
ln2 = Len(navn) + Len(ekstra)

If (ln1 = ln2) Then
    boldIt = navn
Else
    boldIt = navn & " - " & ekstra
    boldTxt ln1, ln2
End If

End Function

Public Sub boldTxt(startPos As Integer, charCount As Integer)
    With ActiveCell.Characters(Start:=startPos, Length:=charCount).Font
        .FontStyle = "Bold"
    End With
End Sub

Solution

This Sub loops through the columns, takes the strings of the two cells, combines the strings and add them to the target cell, while bolding the text of the second cell

Thanks to @Jeeped for the pointers!

Sub boldIt()
Dim pos_bold As Integer
Dim celltxt As String

For i = 2 To 200000
    ' first cell will always be populated - if not - exit
    If (Range("Plan!E" & i).Value = "") Then Exit For

    ' if second cell is empty - take only first cell as normal txt
    If (Range("Plan!F" & i).Value = "") Then
        Range("Kalender!F" & i).Value = Range("Plan!E" & i).Value
    Else
        ' calculate start of bold text
        pos_bold = Len(Range("Plan!E" & i).Value) + 1

        ' create the string
        celltxt = Range("Plan!E" & i).Value & " - " & Range("Plan!F" & i).Value
        ' add string to field and add bold to last part
        With Worksheets("Kalender").Range("F" & i)
            .Value = celltxt
            .Characters(pos_bold).Font.Bold = True
        End With
    End If
Next i
End Sub

VBA merge cells and **Bold** text of second cell

VBA merge cells and Bold text of second cell