This maybe obvious but I just find this - to charge your SugrCube, you need to turn the knob behind the box to ON state, otherwise it will not charge.
After using it for almost one week, the battery indicator in the app shows about 30% capacity so I put it into a USB charger and kept listening. Then I turned it off before I fell asleep. It turns out its battery is still low this morning when I wake up. So to charge SugrCube, you need to turn it ON and charge, just tap it to stop the music if you want it keep silent.