Order Sheet with Automatic Totals only Updating on first initial order

[MikeWalters]

I have an order form that I have made and It works perfectly on my first initial input of the form. 
Parent Table "Order Builder" Has all details for customer Staff and What items are being purchased.

I can select multiple through a REF to a Child sheet called "Cart"

I Have a singular Column that calculates the total cost of all products using this Formula: 

SUM ( SELECT (Cart[Total Cost], Cart[Order ID] = [_THISROW].[Related Carts]))

I need the formula to:

1. refer to child table and select only the rows with matching receipt numbers to Parent table
2. then calculate the "total cost" Column into a sum of all rows created under that Receipt Number
3. Do this for every new order I complete.

Problem I'm faced with is that It only works on the first order and doesn't work again after that.

Please Help Masters!

[Kirk_Masden]

Just some quick thoughts on this issue:

1. It would be possible to accomplish this with a SELECT() expression in a virtual column but if you are dealing with lots of data that could make your app quite heavy.
2. Accordingly, I might consider writing the result of calculations to a third table where I add a new row for every new calculation.

I'm not completely sure if the second approach would work for you.  You may be faced with some additional issues as a result but it will probably reduce performance issues.

[MikeWalters]

That is a good Idea, Use what I have more as a calculator than the actual form.
Cheers I try it and see what happens!
Thanks!