Access VBA not working when already executed previously

I'm pretty new at VBA. I have a form with a text box and a check box. If I open the form and click the check box without populating the text box, I get a message box to enter comments. However, in the same session, if I unchecked the box and recheck it, I do not get the message box to enter comments even tho the text box is blank. How can I get the code to "refire" in the same session to give me the message box to enter comments? Below is my code:

Private Sub Check29_Click()

If IsNull(Me.Text16) Then
MsgBox "Comments are Required.", vbCritical
Me.Check29 = Null
Exit Sub

Else

If Me.Check29 = -1 Then

Dim RS As DAO.Recordset
Set RS = CurrentDb.OpenRecordset("Exclusions", dbOpenDynaset)
RS.AddNew
RS("HW535ID") = Me![HWID]
RS("Excluded") = "Yes"
RS("BOA Assignee") = Me![AssignedBA]
RS("Comments") = Me![Text16]
RS("CheckBox") = Me![Check29]
RS("Date of Exclusion") = Me![Text115]
RS("ReviewID") = Me![Text33]
RS.Update
RS.Close
Set RS = Nothing
Exit Sub

Else

DoCmd.SetWarnings False
DoCmd.OpenQuery ("RemoveExclusion")
Me.Text16 = Null
Exit Sub

End If
End If

End Sub