I had a problem with the unit activity log where I accidentally entered a start and end date for a service project that was a year apart instead of the same day. it did not show up in the log.
But it did show up on individual scout logs, and I was able to go into the activity from there and edit it to the same date. Once I did that the event appeared.
I suspect there may be a display issue in the activity log when an activity has start and end dates that cross months. i reported my issue here but never got a response.