Without more information about how you’ve configured Nginx and the program running behind it, it’s hard to give you an exact answer to your problem. The first place I would look is the value of the worker_processes directive in your Nginx configuration (on Ubuntu it would be in /etc/nginx/nginx.conf). In order to ensure that Nginx will make use of all of the available CPU cores, the value should match the number of cores available. Newer version of Nginx (including the version in Ubuntu 16.04 and above) support auto as a value. This will attempt to automatically dectect the number of cores available.

For more information, see:

• The Nginx documentation for worker_processes and related directives
• How To Optimize Nginx Configuration
• Tuning NGINX for Performance (blog post from the Nginx team)