@romaje: Legacy M7 Mobi, Amazon KF8 Mobi, or both?
(I ask, because reaaaally old M7 Mobis are likelier to have borked metadata and/or no cover).
If you PM me the system log that'll be created in the documents folder by running the
;dm search command from the Homescreen, that *might* shed some more light on this, but I'll *probably* need a verbatim copy (i.e., from the device, not Calibre) of the book file, too
.