Converted files no longer appear on Kindle

Hi,

I don't know why, but all the files converted with Calibre no longer appear on my Kindle.
I can convert them (Epub to Mobi) easily and transfer them via USB but they don't appear on the device.
I never had this problem before. Is this a problem with my device or something else?

I heard that there is a bug with the Amazon covers so I put the device in airplane mode but it doesn't change anything.

Do you have an idea?
Thank you very much.