Hey Roberto
I have almost the same problem, I'm specifying a (constant) level boundary and there is no water coming out of it even if the level is 10m above ground. This appears very strange to me since such a boundary normally should make sure that transfer is happening - in and out of the domain. I guess that in MIKE 21 one should be using sources for this, but this seems a bit misleading when speaking of boundaries.
Let's hope that someone soon has an explanation for this!
Best regards,
Sam