when was the last time they bathed? Different birds don't like to bathe in certain ways, some may love a bath in a bowl and others may hate it and prefer a tap under sink.
If they're desperate for a bath try one of the 2 (if they're not hand tame) put the hoover on, it seems to send birds into bathing frenzy though not all. Or try misting them, get a misting bottle fill it with some slightly warm water and spray it above them so it gently falls on them like a very light rain