EDIT: I believe that the direction of the “flow” can only change on an elbow piece. think of the + shaped pieces as two separate pipes (one going over the other) rather than an intersection
i can see how that makes sense because there are no open ends, but i found this one that also worked https://snipboard.io/F3MfX8.jpg similar setups that leave an open end did not count as finished
EDIT: I believe that the direction of the “flow” can only change on an elbow piece. think of the + shaped pieces as two separate pipes (one going over the other) rather than an intersection