I had this problem a while ago. I deleted a mount and afterwards added it with a new IP. However I noticed that when I used the same name it just kept loading. After looking at it for a while it turns out that the old mount wasn't actually removed. It was still trying to mount it, even though that IP was not accessible. Thus it kept loading.

You can view all mounts that the NFS client wants to mount by writing:

nfsstat -m

A list like above should appear (however there is much more info).

If you notice that one of the shares is stuck you can simply force unmount the folder. Don't worry if your new mount also uses that path, Proxmox will remount it automatically (wink). However it's probably a good idea to stop VM's that use the mount.

umount.nfs -f /mnt/pve/NAME

If you get a device is busy error, try doing a lazy unmount by appending l to the f so you get -lf.