Website Sessions Generating as "Not Set" even with Tracking

[Amsterdam]

Good afternoon, 

I'm experiencing an issue with Looker Studio. We use Looker Studio to track varies data fields for email marketing traffic to a website. Below is a screenshot of email names with session counts. Recently we've seen an large up tick in the "Not Set" category. All of our emails have correct tracking associated with them. I'm not sure if something changed and we need something new. We use Acoustic as our Email Service Provider, and link it with Looker Studio. Any help would be appreciated!