Kolibri studio thumbnails not saving

Operating System (e.g. Windows, MacOS, Linux): windows

Browser (e.g. Chrome, Firefox, Safari): Chrome

Channel (Name of the channel you were working on): Maths grade 3

Description of problem: Kolibri is not saving my changes. I created the thumbnail but it is not saving it at all. This is the 3rd one it is doing it to. I have to re-load it and wait a minute or two then only it accepts it. Please help