I think you are saying that you can see the entrance to the next level of the dungeon but cannot get into the room with the entrance - is that correct?
Try getting close to the room with the entrance to the next level and either 1). Click on the floor inside the room and see if your characters find an entrance into the room, or 2). get close to the room and click on the room in your mini-map.
Sometimes one door to a room will be blocked but there will be another door that will let you into the room. I have only seen one time where a small room (with only one door) could not be entered.
