If you're noticing, though, that emails have started being sent without tracking and you've not changed any settings in Gmail/Mailtrack then there are a number of things you can try to get things working again:
Yes, just refreshing the page can correct the checkmarks a lot of the time!
Delete the cookies for Mailtrack1) In a new tab in Chrome, copy/paste chrome://settings/siteData then press enter.
2) Search for mailtrack.io using the search option provided (as shown in the image below), then click on the trash can to delete the cookies for Mailtrack.
3) Refresh the tab containing Gmail 1-2 times then send an email to test whether the tracking works.
Enable Mailtrack for a Gmail account that you've never used with Mailtrack before
1) Open a new browser tab and go to gmail.com
2) Log into a Gmail account that you are not currently using with Mailtrack.
3) Enable Mailtrack on that Gmail account.
Once the Gmail tabs in your browser have finished refreshing then when you send an email from the Gmail account that you normally use with Mailtrack the tracking should start to work correctly.