What does the output of "df -h" show on the CLI, does it match the values reported in the GUI?
Are you taking the VM memory swap file into account? When you power-on a VM, ESXi creates a VM swap file that is the size of the guest RAM (minus memory reservation, if you've set some) that will be used in case a physical memory contention situation needs pages swapped to disk.
So for example, if you have 60GB of free space on a datastore and you try to power-on 2 VMs with 32GB RAM (and no memory reservation), only one of the VMs will be able to power-on.
A process-swapfile will also be created at power-on time, but it's usually just 100-200MB in size.
Could there be snapshots taking up too much space?
There was only one Veeam snapshot that the Veeam support tech had me delete.