If you are updating the same episode over and over, as many series do, you will not show up in the fresh section. The Fresh section shows your thumbnail only if you have updated a new episode.
However if you have updated a new episode and it doesn't work, make sure to check how long it's been since you have recently updated. In almost no time can a series be pushed down.
But after all of that, it may be possible that there is a glitch in the system. A simple way to fix it is to delete the episode and re-upload it again. The site's not perfect, but that shouldn't stop you from doing what you need to do.