If you´ve sent emails from a device that has Mailtrack installed but the check marks in your sent mail folder are greyed out and Mailtrack says they are "sent from device without Mailtrack"...
.... then hopefully reinstalling the Mailtrack extension on your browser will get things back on track. Instructions explaining how to do this can be found here.
If, however, the green check marks still don´t appear after reinstalling Mailtrack then you should try opening your email account in incognito mode/a new private window to see if the tracking starts working correctly.
Please note that if you are using Chrome then you would first need to allow Mailtrack to work in incognito mode. To do this:
1) Open a new tab in Chrome, copy/paste chrome://extensions/ into the address bar then press enter.
2) Look for Mailtrack in your list of extensions then click "Details".
3) Scroll down the page to where it says "Allow in incognito" then check the option provided, as shown here:
If the green check marks start appearing when testing in incognito mode/a private window then the problem is most likely being caused either by a caching issue or a possible conflict with another browser extension (browser extensions which can sometimes cause problems with Mailtrack include Ghostery, Virtru, and uBlock origin), in which case you should try:
1) Clearing the browser cache (for details of how to do this in Chrome click here, for Firefox click here, and for Opera click here).
2) Testing by disabling any other browser extensions installed on your browser one-by-one, to see if this makes a difference when using Mailtrack in a normal browsing session.
If the green check marks still don't show up when testing in incognito mode then it might be worth testing with a different email account to see if the tracking works. If the issue is fixed when testing with a different email account then you may need to play around with the settings in your email account (enable conversation mode, disable split view preview panes, switch from the new Gmail to classic Gmail then back again...) to see if that helps restore the green check marks.