Maybe it is because you're initializing the score in the same procedure that you're adding it to?
I am assuming the snippet you posted is from the "Check Answer" [or something like that] button's Click event handler.

Here's how I would go about it:

Code:
'Declarations
Private score As Integer = 0
Private userAnswer As Decimal

'...Form Initialization Code...

Private Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
    If Decimal.TryParse(TextBox1.Text, userAnswer) Then
        If answer = userAnswer Then
            score += 1
            TextBox2.Text = score
            MsgBox("Congrats")
        Else
            MsgBox("Incorrect Answer")
        End If
    End If
End Sub