Yep. We need a bunch more information to help.
If the files are local (on a hard disk in the computer you're using to play them) and they're doing that, it means the files were encoded poorly. That's what MC does when receiving a live stream. If a local file does that, it is marked incorrectly as a live stream and so seeking support is disabled.
If it is a client-server kind of setup, we need a bunch more information on how you have it set up.