I'm having trouble understanding the best way to create a dynamic dropdown.

Let's say I have 5 Product categories: A, B, C, D, and E.  Within each category, I have 4 components: 1, 2, 3, and 4.  But each component is unique to that product.  So product A has components A1, A2, A3, and A4; product B has components B1, B2, B3, and B4; etc.

I'd like to create a form that allows our system administrators to edit the products and categories, and I'm currently using a form and a subform for this.  The administrator can select the product from a dropdown field (i.e. pick product B), then enter the different components in a subform (enter B1, B2, B3, and B4).

However, I'm having a hard time creating a second form that allows users to select the product and its components.  I want a lookup dropdown field with options of the different products (from the first form), then a lookup dropdown field with options for the components (also from the first form).  I'd like the component options to filter based on the product selection (when product A is selected, only components A1, A2, A3, and A4 are shown, not B1, B2, B3, C1, D4, etc).  I can't get this to happen because when I attempt to create a lookup field in my second form, it doesn't even list the Product field from the first form as an option for a relationship to create.

What am I doing wrong??  Is there a better way to create this lookup relationship?