A fault tolerant Primary VM can remain in the need secondary state even though sufficient resources are available to start the Secondary VM.
vSphere HA might not restart the Secondary VM of a vSphere Fault Tolerance (FT) virtual machine pair even though there are sufficient resources available.
To restart a Secondary VM, vSphere HA requires that the Primary VM be running on a host that is in the same partition as the one containing the vSphere HA primary host responsible for the FT pair. In addition, the vSphere HA agent on the Primary VM’s host must be operating correctly. If these conditions are met, FT also requires that there be at least one other host in the same partition that is compatible with the FT pair and that has a functioning vSphere HA agent.
To fix this condition, check the vSphere HA host states reported by vCenter Server. If hosts are identified as partitioned, isolated, or unreachable, resolve the problem before proceeding. In some situations, you can resolve a restart problem by reconfiguring vSphere HA on the host that vCenter Server is reporting as the primary host. However, in most situations, this step is insufficient, and you must resolve all host state problems.
After you have addressed any host state problems, check if there are any hosts in the cluster other than the Primary VM's that are compatible with the FT virtual machine pair. You can determine compatibility by trying to migrate the Primary VM to other hosts. Address any incompatibilities that are discovered.