Forwarded port is closed after a couple of minutes

Hi, I've got a problem with my FortiGate-80CM. I've got a port forwarded and open, which is working fine, but the problem is, that it's open only for a couple of minutes (I can't get how long port is open - sometimes it looks like about 5 minutes, and on the other hand it could stay open for 30min and more). I'd like to set that port open until I'll close it maually. I can only suspect, that it may depend from the amount of sessions on that port - there's 0 by the most of time. I've got no idea where to look for any option for that. Can anyone help me with that?