Has anyone noticed this? My Cactus v5's are VERY unreliable when one unit is above the other, i.e. when I have a flash on a flash bracket. When I first turn them on, they'll be in sync for a bit. Then the receiver will stop receiving, or the receiver will display a constant orange test LED. But when you bring them closer to the same plane, they're nearly flawless. I suppose it's possible that the antennas in these put out a dipole pattern like the one on the bottom half of this image (I'm not an antenna engineer but I wish I were):
I was hoping not to have to order a flash sync cable because my photography budget is tight, but I had no choice with this problem.