It is very difficult to tell what could cause DTM tags to not fire without looking into the codes or the web pages 😞
There could be many reason why you don't see DTM tags not firing, you might want to check the following:
1. Check if there is any JS error on the pages which might potentially block the server call.
2. check if the DTM web property is published (if not , this header code will not work)
3. Check if DTM property is configured correctly.
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Hope this helps!
Harsh Kabra | Adobe Certified Expert: Digital Analytics® & SiteCatalyst Implementation & Reporting®