I run a online shopping website, and I use GA to measure ROI of different traffic source, such as meta ads, google ads, etc.

Now, I find some strange orders that should attribute to google ads, but they attribute to not set. When I use explore to see the user behavior details, I find there is no session start event record.

In my opion, session MUST be there, because it’s the base of GA to analytics data.

My question is: how did this happen, and how to avoid or fixed.

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I have already find more than 20 orders have the same sitation