Update 2 rows on another form on submit

Update 2 rows on another form on submit

Hi,
I have a form "movements" and another form "Stock" with different locations.

When user enters 200 for "Item A" in form "movements" I would like to reduce the qty of "Item A" in form Stock for that item and location, and increase the qty of Item A in another location. 

Stock type will be chosen on the movements form. Stock will always originate in the same location and end in same destination.

I tried the following for on submit, on success:
                    
                    if (input.Stock_Type  =  "X")
                    {
                       //fetch stock from origin location of type x
                        for each StockUpdate in Stock  [ID = 943839000000061039]
                        {
                            //reduce values in location by amount input to move
                            StockUpdate.Value = (StockUpdate.Value  -  input.Value);
                           
                       }
                    
                        //fetch stock from destination of same type X
                        for each StockUpdateDestination in Stock  [ID = 943839000000072003]
                        {
                            //increase  values in destination by amount input to move location
                            StockUpdateDestination.Value = ( StockUpdateDestination.Value   +  input.Value);
                        }
                    }
                    //End  if

The above does not give errors but also does not perform the add and subtracts as required. I check my stock sheet after submission.
Any help would be much appreciated. I may not be using the best method to perform this operation and will happily change tack if a smoother way of doing this exists.
Regards,
Robbie