VS 2008 Updating Table Fails

why this update is failing? The table is connected to OK and there are appropriate values in the parameters to the subroutine, and yet the record in the table remains unchanged. I am using exactly the same code (with different parameters, obviously) to update another table and this continues to work fine.

Public Shared Sub SaveMathsSettingOptions(ByVal ID As Integer, ByVal Auto As String, ByVal Repeats As Integer, ByVal Weekly As String, ByVal ClassSheets As Integer, ByVal HomeSheets As Integer)
Dim con As New MySqlConnection(_connectionString)
Dim ret As String
Dim sql As String
sql = "UPDATE centres SET maths_auto = @maths_auto, maths_auto_reps = @maths_auto_reps, maths_weekly = @maths_weekly, maths_class_sheets = @maths_class_sheets, maths_home_sheets = @maths_home_sheets WHERE centreID = @centreID"
Dim cmd As New MySqlCommand(sql, con)
cmd.Parameters.AddWithValue("@maths_auto", Auto)
cmd.Parameters.AddWithValue("@maths_auto_reps", Repeats)
cmd.Parameters.AddWithValue("@maths_weekly", Weekly)
cmd.Parameters.AddWithValue("@maths_class_sheets", ClassSheets)
cmd.Parameters.AddWithValue("@maths_home_sheets", HomeSheets)
cmd.Parameters.AddWithValue("@centreID", ID)
Try
con.Open()
Catch myerror As MySqlException
'MessageBox.Show("Error Connecting to Database: " & myerror.Message)
ret = myerror.Message
con.Close()
Finally
con.Dispose()
End Try
End Sub