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The problem is that the increased ambient light outside is causing interference with the optical slave. A very common problem. Canon and Nikon wireless have the same problem. My guess is inside, if you shined a very bright flashlight on the optical slaves, they wouldn't fire as well either.
You have to position the other flashes so they can be triggered optically by the slaved flash(that usually means closer), or get more slave receivers, or sync cords. I sprung for radio poppers for my Canon flashes, and it is light the sky parted and God smiled down at me!