When sending images from the WhatsApp Business API, you will need to send at least 4 images in succession. If user's conversation view is currently active when the images are received, then album view is unavailable until next visit.
An album will not be created if any of the following are met:
Images with captions
Unread divider - user sees some images, but not the rest
Message from user or other participants if group only
Date header - new day between deliveries