Opens aren't showing up.
The four things to check, in order: pairing, images, timing, and which account sent it.
Sign in at mailviewed.com with the exact Google account you send from, in the browser with the extension. No pairing = no tracking. The extension popup shows which account it's tracking.
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.
The compose window's pill controls each email. If it was toggled off (or the master switch in the popup is off), that send isn't tracked.
Still stuck? Email [email protected] with the email's subject and send time — we can trace exactly what the pixel saw.