Sales tax not calculated on mixed physical/digital product

I have a storefront where nearly all of my physical products are sold with a free additional download component. When I'm selling these products in-state (Maryland), I need to charge sales tax. I have each of the products configured as a Taxable Good; physical products that do not have a download component are getting taxed correctly. The problem, of course, is that the ones which are physical/digital hybrids are not getting taxed at all when it's appropriate to do so.

I'm running Zen Cart v1.3.8a/v1.3.8 -- anyone have any experience with this?

This may have been a legacy configuration issue; the product in question didn't have sales tax charged back in late 2009, but more recent products (now that I have a few more sales in Maryland) are getting sales tax applied, it looks like. So probably a false alarm. If I see more evidence to the contrary, I'll update.