A photo that uploaded but isn't in the album is somewhere specific. Work down the list; the first one that fits is usually the answer.
It's waiting for the host
If the host reviews uploads first, your photo went into their Review section. You'd have seen a note on the page and a toast, Sent, waiting for host approval. It appears for everyone the moment they approve it, and hosts often do that in batches, so an hour's delay is normal at a busy event. Nothing to do but wait; if it's your event, open the Review section and tap Approve all.
You're seeing the preview
On an event that asks for an email, a visitor who hasn't confirmed one sees only the newest few photos with a count of the rest. Confirm your email and the whole album opens. See why an event asks for your email.
The host hid or removed it
Hosts can hide or remove anything, instantly and silently: there's no placeholder. If it was yours and you'd like it back, ask them; a hidden photo is one tap to show again on their side.
It never finished uploading
Scroll to the top of the gallery: a dimmed tile with Tap to retry means it didn't land. See an upload won't finish.
It was added from a different phone
Your dashboard's Uploads shows only uploads tied to your account. Photos added anonymously from another device stay in the album under Anonymous and don't appear on your dashboard.
The page just hasn't caught up
The gallery listens for new uploads and adds them within seconds, without a refresh. On a page left open for hours, or on a flaky connection, it can lag; a reload brings everything up to date.
For hosts: it's in the album but the count looks wrong
The item count on your event card and page counts approved photos only. Photos waiting in Review, hidden ones, and anything in Deleted aren't in it.