Bungie now has factors in place that prevent you from farming like this. Basically there is a cool down factor to them and instead of INCREASING your chances when you run multiple back to back, it RESETS your chances. This has been since about 4 weeks into TTK and about 2 weeks after the launch of 3oC. You had a run of good luck with them prior to this but that luck has officially now run out and this is the way it is.