My function on VBA does give some false negative output

recently I wrote a code in order to check the Social Insurance Number issued in Germany. I have 3 outputs, surname, dob and the given Social Insurance Number. Between 3rd and 8th digits it consists of date month and last two years of birth. however, in some cases, it does return false negative. Can anyone suggest any improvements? I am very new to VBA and coding in general, that's why feel free to check and suggest any improvements. I will post my code, an example where it gives true positive and examples where it does give false negatives. Steinbach 01.12.1991 12011291S533

false negative results for

Akyol 31.10.1993 13311093A017 Voorma 22.11.1995 53221195V018 Köhler 15.10.1997 14151097K056 Xheladini 22.10.1991 65221091X509

The function in Visual Basic

`Public Function firstDigitsSocIn(surname As String, dob As Date, socialSecurityNumber As String) As String
        'defining variables (add gender after surname when data is available)

        Dim dayOfMonth As Integer
        Dim monthSoc As Integer
        Dim yearSoc As Integer
        Dim firstCharSurname As String
        Dim customMadeNumber As String
        'Dim genderCode As Integer            enable when having genders in data set
        'Dim genderCheck As Boolean
        Dim resultFirst7Chars As Boolean
        Dim resultInclGenderCheck As Boolean
        Dim resultFinal As Boolean
        'setting up variables with correct values

        dayOfMonth = Day(dob)
        monthSoc = Month(dob)
        yearSoc = Right(Year(dob), 2)
        firstCharSurname = Left(surname, 1)
        'genderCode = Left(Right(socialSecurityNumber, 3), 2)


        'if gender = "M" am

        'custommadenumber is composed using the credentials of the user
        customMadeNumber = dayOfMonth & monthSoc & yearSoc & firstCharSurname

        resultFirst7Chars = StrComp(Mid(socialSecurityNumber, 3, 7), customMadeNumber, vbBinaryCompare)

        'If gender = "M" And genderCode < 50 Then
        'genderCheck = True
        'ElseIf gender = "F" And genderCode > 50 Then
        'genderCheck = True
        'Else
        'genderCheck = False
        'End If

        'resultInclGenderCheck = resultFirst7Chars And genderCheck

        'resultFinal = resultInclGenderCheck And True 'put Pruefziffer check here as well

        firstDigitsSocIn = resultFirst7Chars

End Function`