Beginner question: unhappy ficus benjamina

I know very little about horticulture in general. I would be very
grateful for any advice you can offer!

I have a "supermarket" bonsai Ficus Benjamina which does not seem at
all well. It lives on a window sill in a decent amount of light, in a
centrally heated house. It will be a bit chilly in that window
overnight, though.

The main symptoms are:

 - it regularly drops leaves and has rather few new shoots

 - the leaves have a brown, rusty appearance and are curling up

 - the surface soil never seems to dry out despite the plant being
placed in a centrally-heated house

Am I watering too much? Too little? Or is the soil perhaps not
well-drained enough?

Any suggestions would be gratefully appreciated. 

Many thanks,

Guy.