Sourdough

Hi I had succes The first time I watch the video on sourdough starter and how to make a boule, it must have been a fluke as now 2 and half weeks of trying and almost 4kgs of flour later I cannot get my starter to float, I told 2 of my friends about the videos and they have been trying for a week and they havent been able to make the starter float either, 2 of us are using filtered water and 1 is using rain water so I doubt if its the water, we are in winter in Melbourne Australia so the temp inside the house is min 8c max 19c weve all been following the instructions closely, Ive even tried