A little follow up on this post: Increasing Disk Size on Existing VM Disk

When extending an LVM system you should also run the following commands:

pvresize /dev/sda3 (replace with your partition)

lvextend -l +100%FREE /dev/mapper/ubuntu--vg-ubuntu--lv (replace with your lvm partition)

resize2fs /dev/mapper/dev/ubuntu--vg-ubuntu--lv (again replace with your lvm partition)