Frequently this is caused by developers either leaving out the:
mm_icon_side_sd=pkg:/images/<filename>
mm_icon_focus_sd=pkg:/images/<filename>
mm_icon_side_hd=<pkg:/images/<filename>
mm_icon_focus_hd=<pkg:/images/<filename>
lines in the manifest file, or leaving out the images they reference, or misnaming those images.
Also, always be sure to add a blank line to the end of your manifest file or the last line might not be read.
Even though these images and lines really only apply to legacy devices and also to sideloaded channels, the channel may not appear on newer devices without them.
- Joel