Well. My sonoline b deffo worked at 9w when my BMI was about 31 and 7w when it was about 26 if that helps? Didn't work so well with the one I lost in January, I was only picking up the hb faintly by 9w and my BMI was only 22 then, but I do think it's worn out, it made a funny whistling noise today lol!
I know the b is 2-3mhz and from what I read the c is only 2mhz, which so what makes me think it might not work so early?