This issue occurs if there are many CustomizeVM tasks stuck in the WorkItem table and cannot be completed. When a provisioning machine is vMotioned during provisioning, it may fail to report back to the correct host causing the CustomizeVM task to fail.
To verify this, check in the Manager Service All.log file for CustomizeVM tasks that have started but have not completed. The Proxy Agent can only queue up a maximum of 11 work items. If 11 of these tasks are stuck, the Proxy Agent stops processing new tasks.