Overwatering is one possibility as a cause of leaf drop on mandevilla. Make sure that when you water, you water enough to soak the root system, but then let the top inch or so of the soil dry out before you rewater. When the soil is continually saturated, the roots of the plant can't get enough oxygen. And make sure that you don't leave water standing in the saucer under the pot; empty any that accumulates after a watering within a half an hour.
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