Use of article

The below are example sentences from Collins Cobuild English Dictionary and Longman Dictionary of Contemporary English. I find it very hard to tell when to use "lack of something" from "a lack of something." What is the difference between with and without an indefinite article?

The charges were dropped for lack of evidence...

There is a lack of people wanting to start up new businesses.

new parents suffering from lack of sleep

Too many teachers are treated with a lack of respect.

comments based on a total lack of information

tours that are cancelled for lack of bookings

health problems linked to poor diet and a relative lack of exercise