Sleep sensor placement

Hello, I kindly ask for your help. I have a memory foam mattress of 180x200 cm from Ikea and a memory foam topper. As previously, I placed the sleep sensor between the mattress and the topper on the bedside. But, unfortunately it does not record anything. I sleep alone in this bed. My bed has slatted frames and I am afraid to put it down there because it might break. I am a tall and massive guy. I did not had this issues with my old mattress (it was also from memory foam in two 90x200 cm pieces, not from one piece). I have reset it on the sensor because of not recording sleep data. Please, what should I do? Looking forward to your answer. Thank you so very much!