Ficus bonsai is shedding its green leaves

I am new to Bonsai 4 days ago i bought a ficus bonsai soon after i brought it home its leaves start defoliating (shedding) i dont know whats wrong and i m worried, can anyone help me out and suggest me a solution ?

Is the soil too dense? Do you water it too much (if the soil was more porous then watering too much should not be an issue)?

Ficus species (especially benjamina) tends to be sensitive to changes in light intensity and so as a reaction it looses some leaves. If that is the case then your tree should bounce back growing more leaves.