Hangman vb.net only first letter showing - vb.net

Question

Basically I generate a random word for example
"Tree" and when I press the T button it changes the label into a T but then when I choose R it doesnt show, can someone else see what i've done wrong?
here is my code
Sub GuessLetter(ByVal LetterGuess As String)
Dim strGuessedSoFar As String = Lbltempword.Text
Dim LengthOfSecretWord As Integer
LengthOfSecretWord = secret.Length - 1
tempWord = ""
Dim letterPosition As Integer
For letterPosition = 0 To LengthOfSecretWord
If secret.Substring(letterPosition, 1) = LetterGuess Then
tempWord = tempWord & LetterGuess
Else
tempWord = tempWord & Lbltempword.Text.Substring(letterPosition, 1)
End If
Next
Lbltempword.Text = tempWord
If Lbltempword.Text = secret Then 'YOU WIN
DisableButtons()
BtnStart.Enabled = True
MsgBox("YOU WIN")
End If
If Lbltempword.Text = strGuessedSoFar Then
NumWrong = NumWrong + 1
End If
DisplayHangman(NumWrong)
End Sub
Private Sub btnStart_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnStart.Click
randomword()
MsgBox(secret)
EnableButtons()
BtnStart.Enabled = False
'Load up the temp word label with dashes
Secret_Word = secret
LoadLabelDisplay()
NumWrong = 0
DisplayHangman(NumWrong)
End Sub
Sub LoadLabelDisplay()
Lbltempword.Text = ""
Dim LengthOfSecretWord As Integer
LengthOfSecretWord = secret.Length - 1
Dim LetterPosition As Integer
For LetterPosition = 0 To LengthOfSecretWord
Lbltempword.Text = Lbltempword.Text & "-"
Next
End Sub
I also generate the random words by doing this.
Sub randomword()
Dim RAND(16)
Dim rng As New System.Random()
For i = 0 To 16
RAND(0) = "Tree"
RAND(1) = "Star"
RAND(2) = "Jesus"
RAND(3) = "Present"
RAND(4) = "advent"
RAND(5) = "Calender"
RAND(6) = "Jinglebell"
RAND(7) = "skint"
RAND(8) = "lapland"
RAND(9) = "Santa"
RAND(10) = "raindeer"
RAND(11) = "Cookies"
RAND(12) = "Milk"
RAND(13) = "nothing"
RAND(14) = "play"
RAND(15) = "sack"
Next
secret = RAND(rng.Next(RAND.Count()))
End Sub