Some Virtual Machines Might Not Power On Even if Their Reservation Is Less than the Unreserved Capacity of Their Parent Resource Pool (2158)
For example, consider a SMP virtual machine with two virtual CPUs, and a reservation of 5GHz, and a host with four physical cores, each rated at 2GHz. Although the host might have sufficient aggregate capacity to supply 5GHz to the virtual machine, each physical core can supply only 2GHz, so a virtual machine with two virtual CPUs could not receive more than 2 x 2GHz = 4GHz. This virtual machine is not allowed to power on. In order to power on, the host must have cores rated at 2.5GHz or faster, so that it can use 5GHz running two virtual CPUs.
You can disable this feature by setting the per-host configuration option Cpu/VMAdmitCheckPerVcpuMin to 0. However, this is not recommended because it might lead to situations where virtual machines do not receive their full specified reservations.