[From FF forums]
I think screenshot explains it all. There's quite big bullet col above the trench, so you can't shoot in the direction I was trying to shoot.
May have something to do with Lewis mounted there (it's nearly at it's direction, above the bipod), but I'm not sure.
[Unfortunately screenshot is gone by now, but the case is that bullet col of bipod of destroyed static Lewis gun is way too much then visible mesh.]