Why was the response rate so low, and how could this be improved for future surveys?

Lisa Harcourt sat at her desk and could not believe the feedback received from the recent employee survey. As HR director for Top Notch Storage, she has always thought that the company had highly engaged and happy workers. However, the data were presenting a very different picture.
Top Notch Storage had been in business for 10 years and had been rapidly growing.
The company employed over 2000 people, ranging from office workers who did bookings, to people who were in charge of placing and retrieving items from storage, to a group of 30 managers who supervised the various employees. Lisa noted that only 400 people (or about 20%) of the workforce responded to the employee survey, but she couldn’t understand why this was the case. The low response rate also caused her to wonder if the information presented was accurate and a true indication of problems at Top Notch Storage.