Activities are stored on the device only temporarily, and may be removed on different occassions, like a firmware update, or when place needs to be made for new ativities, etc. All activities are kept permanently in your Garmin Account, on the server, though. If there is any reason you really need some of them on the watch, you can export them from Garmin Connect Web (in the Original format), and place them into the folder //GARMIN/Activity/.
