Hi Colin,
What you are trying to do is live merge - delete a vm's snapshot for running vm.
This feature might not be supported on some of your hypervisors because of vdsm or OS version.
Can you please share the OS version and vdsm vesrion of the hypervisor that hositng the running vm which you cannot delete it's snapshot?

A quick solution is to make sure you powering off a vm before deleting it's snapshot.

Thanks,
Raz Tamir