I don't know. Last year I was trying to buy bullet train tickets for an upcoming trip to Japan, only to find that the booking website operated only during daytime work hours (Japanese, of course). Then there was me trying to buy train tickets from Mt Fuji area to Tokyo. I could buy them, but they could only be picked up in Tokyo, not Mt Fuji! That was a pretty major wtf, given Tokyo was our final destination.
ofuku-kippu is your friend.
If you mean the Fuji Excursion it's because the Kawaguchiko side isn't the same agency so it's not like JR has machines there. JR's intended solution is digital tickets but I think this doesn't quite work with JR Pass.