Your add to cart button is in a different form than your checkboxes... which means the checkboxes aren't submitted with the add to cart form. They need to be all wrapped in the same form.
When using this technique to add multiple items to the cart with checkboxes on a result page you shouldn't be using the add to cart button interface at all.
1) You can just add a submit button to the results page for the add to cart.
2) Then you add a recordset to the page that filters and returns the rows that are checked based on the checkbox value
3) Apply the "Add to cart from Recordset" server behavior to add all of the rows from the recordset created in step 2
You may want to use a premium support ticket to tackle this for the first time since I can see you are having difficulty and I could walk you through it in a screen sharing session.