The delivery period for automatically-generated print suggestions starts on the 15th day of the month if at least five photos taken in the previous month have been uploaded to an album.
If less than five photos have been uploaded or if the delivery period has ended, you will not receive a print suggestion.
If you do not see any suggestions on the Store tab under "Our Recommendations", you will need to manually select the photos in order to place an order. Please see How do I order prints? for more details.