I had a Ficus benjamina which I allowed to get too cold in a spare room. It dropped all its leaves. When they regrew about 6 months later, the leaves were dull, a matte finish rather than a shiny one. But it was otherwise healthy and flourished until it got far too big and I gave it to someone else. You said your mothers leaves were dull, so I wondered if there was any possibility of a cold shock in the plant's history.