So, sometimes programs can be funky like that. If I log in at 8pm, log out at 8:10, and log in again at 8:25, it will only show new posts that happened after 8:10 when I logged out or closed the browser. That is because the program will register your last visit as between 8 and 8:10, with "new posts since last visit" being those that occurred between 8:10 and 8:15 when you re logged in.
I think the auto "time out" of being logged in is 15 min.
If you didn't close the browser or log out or were gone for less than 15, I am not sure what happened?