Sometimes an entrance can be partly or completely obscured by a wall. Check the auto map, to see if you can see some blue sticking out somewhere. Running around methodically searching with the mouse can also help.
Occasionally the entrance can be a pile of rocks, rather than the usual fountain thingy.
Rarely part of a level can be separated from the rest, which could contain the entrance to the next level. Since you can see sections of the map you can not get to, this may be the case here.
In the main game there is a way to get a summoning doll through a wall and then enter and leave the BF to get your characters there (
armory trick in act 1), but since you are already in the BF, that will not help.
Alt-tabbing out of the game and back can shift character positions slightly, so if you get both characters as close as you can to a wall you may be able to alt-tab repeatedly to get them through. Doing this with other NPCs around can cause problems if they get through a wall and become inaccessible.
Teleporting anywhere (via hex editor)Alternately, you can just skip the rest of that dungeon. You may miss one or more BF quest solutions by not getting to the third level, but completing the BF dungeons is not necessary.