Some iMessage sticker packs need to be manually enabled if you have the Automatically Add Apps option disabled on your device. When enabled, this feature automatically adds any apps to your App Drawer that work with iMessage. You can enable the Automatically Add Apps slider, or manually add apps to your App Drawer by following these steps:
While Stickers have a time and place, they can quickly become annoying if you have an overzealous friend, especially if they place them right on top of the text. Although it isn't readily apparent, there is a way to remove stickers. Tap and hold on the sticker (long press) until options pop up. Select "Sticker Details."