I found that the 7% saline left in the small holes of the bebulizer diverter may be difficult to rinse completely out and it became crystals after drying, which would block the passage of air. It cause that 4ml saline cannot be finished in 20 or even 30 minutes.
What I did is put the nebulizer in boiling water for very short time then take it out to a clean plate for air dry,It works at first couple times but then problems come out again..Now I use 90% alcohol rinse for disinfecting neb ,after I rinsed it under very warm water.If it still doesn’t work, I changed it to a new one.I’m using side stream portable neb cup.They’re pretty cheap.
Anyway if you spend 20 minutes still unable to finish 4ml saline it means your neb is stuffed.