but the white ones seems in distress, as all
the flower heads are hanging as if in need of
water,

Most of the buddleia I've seen (at least the older
ones, with large flowers) seem to 'droop' over, at
least as far as bending in the middle. If the leaves
look fine (not limp and drooping), I would tend to
think that it's not necessarily in distress, just that
the weight of the flower is causing the drooping.

Tracey