When your galleries are set to "Manual Fulfillment," the download set is paused by default! This gives you time to confirm the order, retouch the files and re-upload the new images. Then, make the download "live" for your client, and they'll get notified that their download is ready.
To set your Download Set to "Live," you need to:
Go into the Gallery
Filter by the Download Set
Click "Live" on the toggle next to the set.
You can choose to email your client after you turn the Download Set to "Live" to send them the link to the download set.