Spam and tabs
This is the most common cause. Ask your client to check Spam and, in Gmail, the Promotions tab. The email shows your name as the sender, so they should search for that, not for the word Picvelo.
The client's address
Open the client page and read the address letter by letter. A typo in the domain is the second most common cause, and it looks exactly like a delivery problem: the panel says it sent the email, because it did.
When the client's mail server rejects an email, the client gets a red Address rejected badge, and you get a notification in the bell and an email. Fix the address on the client card and send the gallery again.
An earlier unsubscribe
If the client once clicked unsubscribe, automatic emails don't go to them. The gallery email you send yourself still arrives, but the deadline reminder and the nudge about an unfinished selection don't. Only the client can undo the unsubscribe, with the link from an old email.
Missing password message
If you saw the message Cannot send: this client has no stored password. Set a new password in the client profile and send again. when sending, the email didn't go out at all. Set a password in the client profile and send again.
A trace in the panel
The activity log records every gallery send, and the gallery moves to the Sent stage. If that trace is missing, the send failed and you should try again: in the album, click Gallery settings, then Send email.