Hi @Cocolobo,
I would definitely start with what @Byrne C already mentioned. But if you want to go further and see your historical performance, you can build a Custom Flow or Campaign performance report and see where the inflection changed (from both Campaigns or Flows). You can learn how to do that here:
Also, if you are currently working with Klaviyo Partner, you can also ask them to run an “Executive Business Report” for you that can also provide more historical report history and a high level view of all the Campaign/Flow performance in the last 24 months.
If you have access to Marketing Analytics, I would also look at how your various RFM groups are changing over time. Or the cohort analysis to see how each “cohort” of customers in the past performed relative to the new ones.
Perhaps there is some fatigue or decline of your more active and engaged customers. Or perhaps, you’re not growing enough new customers at the same rate as before. This one is harder to pin down, but perhaps your new customers/prospects aren’t as ideal (or have the same brand affinity) as your previous higher performing customers in the past.
Hope this helps!