With fast provisioning, we will use the same Datastore (NFS in your case) which has the existing linked clone tree until you hit the red threshold for free space.
This may not be practical with fast provisioning, as once you have 3GB left ... that's not a lot of room for linked clones to grow.
If you want to force the issue, increase the Red threshold to something significantly larger on the affected datastores. e.g. 25% of capacity.
this will trigger placing NEW vms onto different datastores.