First, ensure that you're not simply dealing with an issue related to the nature of AgencyAnalytics "All Time" time period.

If you're not looking at "All Time" data, and you've actually configured a custom multi-year time period, but you're not seeing data older than 26 months or so, then you're probably being impacted by Google Analytics new default retention period.

In May of 2018, the European Union implemented the GDPR, which is a sweeping suite of regulations impacting Internet user privacy and data retention.

In May/June of 2018, Google notified all Google Analytics' users that they would be archiving and deleting any data older than 26 months unless users specifically opted in to have their data retained longer (or indefinitely). 

If you didn't see that message, and/or didn't choose a longer data retention time, then any data older than 26 months has been deleted by Google.  

This has nothing to do with AgencyAnalytics; you'll see the same thing if you log into your account at Google.com/Analytics.

AgencyAnalytics is only able to display the data that currently exists within your native Google Analytics account. If the data doesn't exist in your GA account, we can't display it in our platform either.

This blog post from Elite SEM summarizes the issue nicely.

For further assistance with this issue, please consult the Google support forums, or contact Google support if you have partner access.