I've been playing with the Flexible Gate example. I created a 2D channel and hinged it to the upper boundary, then added in/out layers and it seems like it's working.
However, when I try to make the channel height and the gate height smaller, all the floating objects float freely and eventually leave the domain.
I created new id's for each block thinking that would be the problem but it did not help. When I run the same xml file with a bigger domain it works. I don't think it's normal but I might be missing something.