Without seeing your project it is difficult to say. But two common ways of solving physics problems:
1. Increase the size of your objects. If increasing the size of the cube is not possible, increase the thickness of the plane below. Like, if the plane is only 0,1 meter, try 1 meter.
2. Increase physics FPS from 60 to 120 for example. PROJECT > PROJECT SETTINGS > PHYSICS > COMMON > PHYSICS FPS