I have only just realised that I have been doing it wrong for years and complained how long it took.
The correct way
1. Keep the mask etc clean, wash in hot water every day and air dry.
2. Pour the nebules directly into the reservoir, not down the tube.
3. Now takes about 10 minutes to clear 500 micrograms. Sorted.
I assume the liquid is sticky so by pouring it down the tube it sticks to the sides. This builds up if not kept clean.
Hope this tip might help someone.
JJ