Recently is only programmed to omit really small files (500 px) or below that 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.