Updating an order - no download button added

I've had several orders recently that I needed to correct where the customers deselected the default downloadable format and didn't select a new one. I add the product to their invoice, selected the correct format attribute, set the price to 0 and updated the order. However, when they log into their account, they can see the formats but there is no download button.

My solution so far is to email these products but I allow up to 10 downloads over an infinite time period in case they lose their files. I don't want to have to re-email items.

How can I add a downloadable product to a previous order and have a download button show up.

(And yes, I know I should upgrade. I've spent weeks trying to do that and it doesn't recognize my database.)

TIA - Lindee

Something must be wrong with whatever custom code or addon you've got that you're using to add products to the order ... as in, it's not also properly adding the download attribute part correctly.