Hubble Roo Mode D'emploi page 15

The baby's dynamic level will affect the heart rate. In order to reduce
interference, please try not to touch the device when using it. If you
can't find the heartbeat, don't worry, sometimes it's normal to be a
bit difficult. Familiarity with the process will gradually improve. The
position of the baby, the dynamics and the distance of the fetus from
the placenta can affect the effect.
Operating your Roo 15