Question

how to solve notification problem

Answer 1

Its not a problem, its battery optimization done by oneplus to save the battery. 
Solution
Go to notification bar.
long press the battery bar.
then select 'battery optimization option'.
then select each app , you want quick notification from like whatsapp, facebook, messenger,snpachat, instagram or any other social media apps and select 'do not optimize option.'
done.

Answer 2

To solve notification problem

Go to profile

There is notification settings

And set your notification settings