Why is location not consistently detected?

When Vocus.io captures a tracking signal (email open or link click), it seeks to determine the geographical location of your email recipient (country/city). In case the pixel tracker was loaded using an image proxy, there will be no way to determine the location. More context below.

How email tracking works
You can learn about it here. The short answer is that Vocus.io inserts a pixel (i.e. small image) inside your outgoing emails. When the email is opened, the image is automatically loaded, and the IP address of your recipient is captured.

What is an image proxy?
Some email clients, specifically Gmail Web and Gmail Mobile, load images using a proxy server. This hides the real IP address of your recipient. As a result, you are unable to know their location or what device (web or mobile) they are using. This is industry norm; all email trackers (i.e. other extensions) behave the same. You can learn more by searching "Google Image Proxy" in Google. 

Does this affect link tracking?
No. Link tracking should capture the real IP address of your recipient, and therefore you should expect to see the location if found in the location database.