This could be a couple things:
The Recently app wasn't given permission to access the iPhones' photos.
Go to the iPhone's settings app.
Scroll down until you see the Recently app icon.
Select Read and Write under Photos.
LOW RES PHOTOS
Recently is programmed to omit really small files (500 px) because they won't print well. Usually those are ultra compressed from the web or from a text. Last thing we want is pixelated print outs in a magazine. Otherwise all photos present on the phone should be showing up.
Photos taken on a DSLR, and transferred to an iPhone might be too large (files sizes can exceed 10-20MB) for the app to handle.
If using DSLR photos, best to size them down to standard iPhone photo sizes prior to using Recently.
Somewhere between 3000px x 3000px to 2000px x 2000px will work well.