Lab Management has a lot of moving parts, especially if you are using SCVMM based environments. All the parts have to communicate if the system is work.
One of the most common problem I have seen are due to DNS issues. A slowly propagating DNS can cause chaos as the test controller will not be able to resolve the name of the dynamically registered lab VMs.
The best fix is to sort out your DNS issues, but that is not always possible (some things just take the time they take, especially on large WANs).
An immediate fix is to use the local host files on the test controller to define IP address for the lab[guid].corp.domain names created when using network isolation. Once this is done the handshake between the controller and agent is usually possible.
If it isn’t then you are back to all the usually diagnostics tools