Whew lots of good info! New here and been looking for a solution to my problem, maybe someone can help. i have d300s, sb900 master, sb600 slaves with px transmitters and receivers. I have a misfiring problem...only with the 900 master on camera. if I take the sb900 out and use the popper transmitter hotshoe mount with pop up flash, it fires 100%. For some reason with my 900 mounted as master, the misfires return. Been on the phone with popper, no luck. Note: with no poppers, shooting IR only with the 900 works perfect, its just 900 with poppers no worky! Ugh! Tried moving Xmitter around, even had popper send me new transmitter, not the prob... Stuck, anyone else heard of this, I'm sure its something silly...thanks in advance.