MS Access 2013 Visibility Issues

I have searched, researched and Googled this until I got so frustrated that I signed up to ask this question to the experts :)

I have a checkbox which is determining whether a subsequent field should be opened or not. It more or less works until you reopen the previous record and what should show up based on the code only shows up as a checked box without the field that should be showing(make sense?)

Here's what I have done:

Private Sub IS_SETUP Click()

    If Me.IS_SETUP = vbTrue Then      
    Me.SUPPLIER_NUMBER.Visible = True
Else
    Me.SUPPLIER_NUMBER.Visible = False
    End If

End Sub

What am I missing here? I want what's "checked" to show up in the already filled in forms as well as the checked box. I don't want to have to uncheck then recheck the box for the text box to show up again.