It might be that you have defined it wrong. You should define it as the generated code does:
Return New StoredProcedureCall(dataAccessProvider, "[sotaas].[dbo].[AirPlusJournalCheck]", "AirPlusJournalCheck") _
.AddParameter("@Period", "VarChar", 5, ParameterDirection.Input, True, 0, 0, period) _
.AddParameter("@SalesLedgerDatePlus1", "DateTime", 0, ParameterDirection.Input, True, 0, 0, salesLedgerDatePlus1)
(In RetrievalProcedures.vb).
If this is what you are already doing, the please post the proc call def.