Can you post logs?
How much free space is on each datastore in the datastore cluster, and specifically the one where the VM is currently located? A cluster is just an abstraction to vCLoud ... it'll try and use the existing datastore.
when you try to add the disk, we may check to see if we can fit an entire copy of the vm (not just checking for the newly requested disk size).
hence, you may need much more space than you actually do require just to add the disk.