Things that remind them of the last however many places that erupted in gunfire flare birds. Whether it's an anxious hunter's smiling face, a blind that looks like a blind or a dog that's obviously that, birds that have had the course are going to be leery of it.
Dogs swimming or even running around in the decoys can attract distant birds because they apparently appear like birds, themselves, from that distance. But let close incoming birds that were ready to land with a swimming dog see it climb up on the shore or a dog ramp, and they're probably going to haul arse if they've been under the gun much. And we see much the same field hunting: Pup running back to us with a retrieve is very likely to turn distant game our way, but if birds arrive before he's settled back in, apparently landing with us, and they catch him standing, pressured birds are apt to be bumped by the dog that reminds them of dogs with guns. (How tolling dogs get brought into these discussions beats me, because that is a hide and seek game, with the toller appearing and disappearing, that catches the curiosity of distant birds rafted on the water and slowly draws them closer over time, as their curiosity mounts and they gain confidence that the toller can't harm them.)
It's been my experience that it's best to have the dog where it can mark most falls but to also keep it largely concealed, or at least not sitting out in the open where they're most apt to remind birds of scenes that shot at them. So while I'm good with Pup sitting up where that's needed to see out of its cover, I'll want him laying down and perhaps be covered up with something more natural appearing if he still contrasts with his background.
If you think I'm wrong, you might be right.