If you’ve changed your video preview length, that will only apply to videos that you upload going forward -- it won’t retroactively change previously uploaded video previews. So for example if you set your preview to 15 seconds in length in February, and then upload five videos, and then change the preview length to 30 seconds in March, those five videos you uploaded in February will still have a 15 second preview length.
You can amend this by adding a custom preview to those videos in your media center, though.
Here’s how:
Click the second button on your bottom navigation menu -- that is your media center!
Upload your preview & your full length video in the media center.
On the full length video, click "Manage" and then expand "Other Actions."
Select Choose Preview.