There are four common reasons, and they look similar from the outside. Work down the list.
You're being sent to an upgrade page
If clicking Agency Analytics bounces you somewhere else, Analytics isn't switched on for your user.
Your account owner can enable it in Settings. It's a per-user setting, so being able to see other parts of aOS doesn't mean you have this one.
You haven't connected your accounting yet
Without Xero or QuickBooks connected, Analytics still works, but it shows a baseline view built from your client and service data rather than your actual books. Numbers appear, they're just estimates rather than accounting figures.
Connecting Xero or QuickBooks replaces those estimates with real ones. See Connect Xero or Connect QuickBooks.
You've connected, but haven't run the history sync
This is the most common one, and the least obvious, because connecting and backfilling are two separate steps.
Connecting gets you the current month. Your history is a step you run yourself: Agency Analytics → Setup, set the date you started using your accounting tool, and start the sync. It works through your months one at a time with a progress count.
Leave that page open while it runs. Navigating away stops it partway, which is why Analytics sometimes shows a few months and then nothing further back. Returning to Setup and running it again resumes where it stopped rather than starting over.
One chart is empty, but the rest look fine
That's usually the revenue-by-service breakdown, which needs your clients tagged to services before it can split anything.
Go to Agency Analytics → Setup → Clients and tag each client to the service they're on.
Still empty after all of that
Check Settings → Integrations still shows your accounting tool as connected. A connection that was removed, or access revoked on the accounting side, stops new figures arriving without any other sign.
If it's connected and the history sync has finished, contact support.