There are a couple of reasons why your photos might be missing.
They have not been found on your Cloud storage. This means they might have been deleted, or their upload was interrupted. If you have your photo backup set, please make sure you are connected to the internet and your photos are all uploaded before you close the app. We have a tutorial for this.
Another possibility is that the photo is not correctly imported to the app. This might be the case when you hit your phone storage maximum. If this might be your problem and you have not enough space on the phone, try rebooting the phone or clearing the cache (temporary files) from apps that store most content. For more tips see this tutorial.