Hi @acodyjr, @edang03 and @Jennifer_Rothrock,
I'm assuming most of you are using the default munchkin snippet and depending on your particular use case, it might need to be adjusted.
In order to troubleshoot this, you need to:
Check whether the munchkin snippet has the correct placement.
Whether the snippet is firing correctly.
What cookies are being created or for that matter not being created. If a cookie is not being created, try to figure out why that is the case. Cross-domain issues? Top level domains with just 2 letters (.jp) or co.uk?
Check to see whether you might have some kind of DDoS protection in place that stops outgoing calls to external APIs.
These are just a few things that come to mind without actually seeing any of your pages.
In regards to using GTM... The main benefit is that you have a single place to insert and control your scripts. Set firing rules. Insert necessary scripts across 1, 10 or 1000 pages. If you need to change something in a script, you do it once and it's applied across however many pages you need/want.
Last but not least... can you elaborate on your notes about Unbounce form playing nicely with Marketo? Are you referring to the native Unbounce integration that pushes form data to Marketo?
If you are referring to associating anonymous to known leads... This needs to happen with an API call to Marketo.
Feel free to reach out in a direct message if you need more help.