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 approval
If the host reviews uploads first, your photo went into their Review. You'd have seen a note on the page saying so, your photo lit on the album's sheet of what waits, and a count on the round button beside Add photos, which opens Your uploads, where it says Developing. 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 Review from the event page and tap Approve all.
You're seeing the preview
On an album that asks for an email first, 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. On an album that asks for a photo first, the preview lasts until you've added one of your own; see a photo first, explained.
The host turned it down, hid or removed it
Hosts can reject an upload waiting for review, and hide or remove anything in the album, at once. The album keeps no placeholder, so other guests see no sign of it. At an event where the host reviews uploads, your own list says so: the round button beside Add photos opens Your uploads, where a photo of yours that isn't in the album is marked Not approved, with no reason given. If you'd like it back, ask the host: a hidden or rejected photo is one tap to show again on their side.
It never finished uploading
If it didn't land, the sheet that opened once the rest were done listed it, with its reason and Retry; if you closed that sheet with Not now, add the photo again. 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 under an unverified name from another device stay in the album, marked, 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.