Auto deployment of agents get stuck in 'Pending' state if the coordinator performing the deployment was added after enabling Auto Deployment. After that none of the deployments can proceed until the 'Pending' state is cleared.
The following message is also reported in the Coordinator log file:
Message: Exception getting a remote deployment item.
To fix the issue
1. First clear any pending agent installs or upgrades from within the Install or Upgrade scheduling window.
2. update/reset Auto Deployment credentials stored under Deployment | Auto Deployment | Set... (Please note the change is accepted and is confirmed by a message balloon that pops up stating Forest Credentials updated)
After this auto-deployment will work as expected.