The alert bar and Diff Analysis bar in team center are alerts that match the node and the alert has "Propagate to Team Center" checked.
If you had the Sample management module active, this may have populated the alert and diff analysis bars. So here are some guesses on why they are not being displayed any longer:
1. Application / Server changes
If you are in the application perspective, maybe the application that was there before has been disabled or renamed.
If you are in the node perspective, maybe the agent on the JVM is no longer active, or the JVM has stopped.
In either case, there isn't any matching criteria that would associated an alert to the team center node.
2. Management Module Change
The management module that has alerts that match the team center node is no longer active
The agent expression on the MM has changed so the alerts no longer match the team center node
The agent/metric expression on the metric grouping, related to the alert has changed and no longer matches the node
3. Other wild guesses
Very much doubt that if you see other nodes with the alert bar and diff bar, that it would be APM issue but here are a few guesses.
The APM database that stores the various Team Center data might have an issue
The EM-Collector that the agent is reporting to is metric clamped and can not store the application metrics
The EM-Collector is agent clamped and there isn't another collector for the agent to report to that has capacity
My suggestions are to trace the application down to the average response time, define a new Management module, define a new simple alert to the specific application average response time. Set simple thresholds on the alert and check both the management module and alert to be active. Set the alert's thresholds, observed period, periods over threshold very low so it will go into danger and wait a bit for the team center to display the alert and diff bar.
Check to see if there is any reported metrics on the application. Maybe the application is no longer taking traffic and the metrics have fell off.
Define a diff alert for the management module to see if you can get it to activate.
If you can get a new management module/alert/diff to display on the node, then really good chance someone changed an alert, or management module so the team center node is no longer related to an active and propagated alert/diff.
Hope this helps,
Billy