Download Button Missing After Ordered Digital Photos Replaced

I have a collection set up for clients to select 5 images which are included in their session fee. I have it marked to not allow download until I replace the images. The whole process worked fine -- I went into the order => manage photos => replaced photos. But then after that, when I click "View customer order on website", I can see the replaced photos, but there is no "Download" button and no "Download Pictures in a ZIP File" button at the top. So there is no way for my client to download her images. I have confirmed that the item used in the collection is set up as a download. I have been processing many orders the past few months (using a different price list) with replacing photos for orders of digitals with no issues. The only thing different this time is the cost is $0. Please advise what could be causing this issue so my clients can download the digital images they paid for. Thank you!