Hi Assj,
I also have the ww digital scales, also on ceramic flooring.
Mine jump about like you wouldn't believe, I can literally weigh myself 3 times and get 3 different readings! (and I'm talking a good few pounds!)
I have tried new batteries, moving them to a different room and standing them on a 'cut to size' piece of marble worktop, all to no avail!
Mine weigh a good 3lb heavier than the scales at the G.P.'s so now I stick to going to boots once a week!
They still call out to me everyday though and like a fool I climb onboard to hope and pray that they show the slightest downward movement.
I take it as a form of domestic abuse. 'My scales lie to me'. Im sure there must be a group for this? lol.