I have an SD card that is full. Despite deleting files on it it is still full.
I only have this problem in Ubuntu.
What is going on?
Did you delete the files in using the GUI Files tool? If so, they are probably moved to the Rubbish Bin rather than deleted.
Open Files, right click on the Rubbish Bin and choose Empty Rubbish Bin.
On removable media, Ubuntu creates a folder called
.trashes which works like the trash bin does on the rest of the computer. You can bypass this behaviour in your file manager's settings, but I normally just use Shift+Del to permanently delete the files without having to worry about the trash bin.