Hi James:
I was going to ask you if you were in menopause (which would account for it), but that's clearly not the case.....! lol.
IMO you're clearly doing something right, or you wouldn't be losing at all - and you've come SO far; really, an inspiration to us all! Have you maybe lost inches around your waist on the days you don't lose on the scale? Doing a lot of weight lifting? On days when I don't lose a pound, I try to just focus on the tape measure instead, failing that I try not to let the scale wreck my day. I know it's difficult, but it can be done.