That can happen when there are deform keys for attachments that are not visible because they are not in the active skin (though they will show if they have a linked mesh visible which inherits deform). This is done because there could be many skins, which could mean many deform timelines.
You can use the tree view setting
Show all skin attachments which makes the dopesheet show all deform timelines, even if the attachment is not visible in the current skin.
You can also use
ctrl+H to show any hidden bones/slots. When bones or slots are hidden, their timelines are also hidden.