Digital Download Not Showing Up For Client

I've done this a million times before, so I'm not sure why it's not working now.   

But, I had a client contact me today because when I sent them the "send email to notify photos are ready to download" email, the email they receive doesn't have the 'download' button.  I'm sure it must be something I've set up differently in this particular pricing structure, but I don't know how to fix it!

Thanks :)

The email they receive won't have a download button. It will have a link to the order and download it from the order.

I meant to update this earlier to say I figured it out.  Yeah, it didn't have a download link on their order either.  But, I created the download as an 'add-on' to a package and I didn't select the 'download' option for the item.