Hello,
QEMU have a locking method for images in use if you use a shared filesystem which supports locking. The locked files need some time to expire as NFS server gives some time to NFS clients (hypervisors) to recover in case of incident.
You can disable locking support on mounted volumes on hypervisors by adding nolock option at mount option at fstab or decrease these timeouts on NFS server (check --lease-time seconds and --grace-time seconds options).