does the configuration of the sensor bolt appear to be the same as the one it is replacing (diameter, thread pitch, taper, length, etc)?
is it too long and jamming up against something in the transmission that will not allow it to thread in?
if you suspect that, try poking a punch of screwdriver into the threaded hole and see how far it will go in, and compare that to the length of the sensor bolt.
any burrs or thread damage or defects to either the sensor bolt or the transmission case threads?
other than that..............