"On my Mac" e-mail content and attachments won't load on Mail application

I have created folders "On my Mac" to organize e-mails from different accounts using the Mail program.

Often, however, the content of these e-mails and their attachments, do not load.

It merely appears blank with the sender e-mail and e-mail title but no content.

Why this this happening ? and How to ensure it stops happening in the future?

Is there any way to recover the lost content?

Thank you in advance //