ext4 filesystem not recognised on new LVM volume

[John Lane]

I have this weird problem occurring today.

I have a disk that has LVM on it with a number of logical volumes
containing ext4 filesystems.

Grub has been able to see this disk and detect the ext4 filesystems
without issue.

This morning, I added a new logical volume, formatted it ext4 and put a
system on it.

Grub does not recognise the filesystem on the new volume. It sees the
volume fine but that's all.

The new volume works fine on any Linux system I connect it to. Grub
still recognises the other volumes' filesystems fine, it's just the new
ones.

I'm puzzled. Any suggestions as to what I can look it?

Thanks,
John