I am using events to enrol contacts in a Marketing Automation plan, and it works fine in XP Single topology, but in an XP Scaled topology (Azure PaaS) the contacts are not being enrolled.
The plan is set to Active and has not expired. I have verified that the contact has the event in its Interactions data (via xConnect OData API) and I can see the contact in the AutomationPool database table at the time the event was triggered, but the contact remains in the pool and never gets enrolled. The value in the Attempts column for all contacts is 0. There are no records in the other Marketing Automation database tables.
The xConnect collection, xConnect search, and Marketing Automation Operations Azure app services are not throwing any errors in Application Insights.
What might prevent a contact from being enrolled in the plan ?
Environment: Sitecore 9.1 Update 1, Azure Search, MS Azure PaaS XP Scaled.