Well... Where does it live - in or out, in what kind of mix, and how much water does it get how often? What about lighting and other conditions if it's indoors (where it shouldn't be)? Can't just guess at things without any information :-).

my cotoneaster is also losing its leaves... its autumn here and i give it water twice a day keeping the soil moist and immersing it on sundays once a week for 5 minutes. its a small tree.