Stored Procedure with update statement not functioning
===

I have created a very simple stored procedure and verified that the query that needs to be executed is a correct/running query. Yet, once stored in the SP, it does not do anything apart from returning '0': 

    USE [test]
    GO
    SET ANSI_NULLS ON
    GO
    SET QUOTED_IDENTIFIER ON
    GO
    
    ALTER PROCEDURE [dbo].[GenerateVolume] 
    
    AS
    BEGIN
    
    	SET NOCOUNT ON;
    
    	UPDATE test.dbo.[c500 CP_Simulation]
    set GeneratedVolume = AvgFillRatePerDayPerCP +[dbo].[fn_InvsNorm](RAND(convert(varbinary, newid()))) * StDevFillRatePerDayPerCP
    where test.dbo.[c500 CP_Simulation].OrgVPnr = test.dbo.[c500 CP_Simulation].OrgVPnr
    
    	
    END

No errors are given... Please give me a tip :)