Huss
Veteran
The M9 can have issues with older camera batteries when using flash. Blank frames will result, so be sure to have a newer battery in the camera indoors.
My M240 does that too. With fully charged batteries - OEM not aftermarket - I get unsynced shots more often than not. Black images. Happened with my M-E too, and at the time I thought my 24d flash was defective (I had bought it used) so I bought another used one. Nope, it's the camera battery for some reason.
Of course both flashes work pefectly on any other camera I try them on, no matter what brand. Including an M7, Nikon F6 , Minolta Xk etc. Anyway, if I need a digital camera w flash I use a D750 with SB500. Works perfectly everytime.