Bundle product rule

I am trying to make a rule that calculates the overall price of a number of products. To do this, I have made a new product type called "Bundle", and I reference "Retail" products within a product reference field. (I have tried using the Product Bundle module, but it doesn't work properly with my site, so I wanted to make a rule instead)

I have made the calculation work by looping through this field and then adding the "list-item:commerce-price:amount" to the line-item total.

The problem I'm having is that it isn't calculating the tax on the 20% VAT products. I have tried creating separate product reference fields and then adding these totals together, but it still won't work.

Can anybody offer me any advice on how I would make this work?

Thanks.