Opens aren't showing up.
The four things to check, in order: pairing, images, timing, and whether the compose pill said Tracking on.
Sign in at app.mailviewed.com with the exact Google account you send from, in the browser with the extension. No pairing = no tracking. If the extension popup says Sign in to MailViewed to activate tracking, that account isn't paired yet. From extension 0.7.23, the label next to Send in Gmail reads Sign in to track when it isn't. Signed in to MailViewed with a different Google account than the Gmail you send from? Open Switch account and sign in with the one you send from. The plain sign-in page can't do this: it takes you straight back to the dashboard you're already signed in to.
Tracking registers when the recipient's mail app loads images. 'Ask before displaying external images' or a strict corporate policy means a read can pass silently. That's a physical limit of pixel tracking, for every tool.
Loads in the first seconds after sending are deliberately ignored — those are your own compose window or server prefetches, not your recipient. A first open through Gmail within a couple of minutes of sending is held too, because the Gmail phone app downloads images for its notification before anyone reads it. It counts once they click a link or reply, and any open after that window counts as usual.
The compose window's pill controls each email. If it was toggled off (or Track new emails is off in the extension popup and you didn't click the pill on), that send isn't tracked. Nor is a send made while the pill said Paused · other tracker, or one MailViewed told you went out without tracking because it couldn't reach its server in time or tracking is paused on your account.
Still stuck? Email [email protected] with the email's subject and send time — we can trace exactly what the pixel saw.