Parthenosisus Henryanna shedding leaves

We have three Henryannas in three different spots in our north-facing London garden. They are between 10 and 13 years old. They have always done very well during the growing season and this year were thriving until after July 7 when all of a sudden they all lost a huge amount of leaves simultaneously. They had been watered during the hot weather. We fed them Maxicrop seaweed extract (diluted for mature plants). It is possible that may have prevented further loss of leaves for a few weeks. In the last couple of days, however, they started shedding leaves again. This has never happened at this time of the year since we planted them.

Can anyone think what might be happening and suggest a solution? 

Many thanks in advan