Approval pending on your side
The most common cause. Project shows "waiting on customer approval" on the previous milestone. Approve and the next starts.
Missing context or asset
The agency team flagged a blocker (missing brand asset, missing CRM access, undefined audience). Look for blockers in the project sidebar.
Scope creep
You added requests mid-project. Each added scope shifts ETA. Either accept the new ETA or split the additions into a follow-on engagement.
Underestimate
The original scope was off. The account manager flags this with a revised estimate and a meeting request. Resolve fast — partial work stalls without a clear path forward.