Web Client - vm user role can power off vm but cannot power on

I have two users that can in the vm user role (sample) who can power off any VMs but cannot power on any.  When they power on the VM, they receive the error below.

"

The "Power on virtual machine" operation failed for the entity with the following error message.

Virtual machine cannot be found.

"

However, when the users tried to power on the VM using the vSphere client on Windows, it works just fine.  Could this be a bug???

Thanks,

Alex