Hard to say without having access to anything. Not sure what site, don't know your Analytics property IDs, don't know your measurement IDs, etc.
Some things you could check though... Look at network activity from a browser and first see if making the logging requests to https://www.google-analytics.com/g/collect
if it is, make sure the tid matches your measurement ID. If that checks out, then the system is trying to log it to the correct property/data stream. That's a good first step... then you can figure out either why it's not trying to log or why Google Analytics isn't showing what was logged.
I'd also check the obvious like any console errors in the browser.
Have you made any major configuration changes (like changing the data stream or measurement ID) for the property?
You could also just disable serving the Google Tag Manager JavaScript from your own domain and see if it's an issue with that.