Cannot change lock screen wallpaper in Ubuntu mate 20.04

I installed Ubuntu mate 20.04 few days ago. I had changed my wallpaper,but after some days i couldn't change to some wallpapers but some wallpapers which i had previously applied work still now.

Suggestions given are accepted thankfully

Can you give us a few more details.

Are you trying to change your desktop background or are you trying to change just the lock screen background?

When you changed your backgrounds how did you do this?