Touch kobo only able to remember what's on the device

I have 110 books and comics on the kobo touch. I generally sync when i add something new. i'm not sure if this is necessary. Sometimes I get the message sync cancelled. At one stage the message kept appearing. Eventually I chanced my arm and disconnected it without getting the "ready to unplug" message. I then tried to reconnect it. I had difficulty. but eventually got it. I tried syncing again, and when the message to eject came up. I did. All the items i put on the device seemed to be there. But what's happening is that the device seems to forget most of the items on it. It will be fine at first, and all items will appear. But if i choose to turn it on 5 or 6 hours later. only about 5 or 6 of the items will appear.

update.
I usually power it off. This time I'm going to leave it on sleep overnight and see what happens tomorrow.