There are a couple of reasons why you might not see this option:
There's a 200 photo limit in photo albums.
The Add More Photos option disappears from an album when you reach the 200 photo limit. You can create a new album or move photos from one album to another to make space for new photos.
Some albums have a different method for adding photos.
Certain types of photo albums are created automatically. You won't see the Add Photos button when you're viewing these albums:
Mobile Uploads: This album is made up of all the mobile uploads you've added. To add to the album, upload a photo from your mobile device.
Profile Pictures: This album is made up of all the profile pictures you've added. To add to the album, change your profile picture.
Wall Photos: This album is made up of all the photos you've added to your Wall (timeline). To add to the album, click Add Photo/Video at the top of your home page or profile (timeline). Pick the Upload Photo/Video option, then select an image from your computer and share. The photo will appear on your profile and in News Feed and will be saved in your Wall Photos album.
Cover Photos: This album is made up of all the cover photos you've added. To add to this album, change your cover photo.