You should use the unhide-tcp tool :

unhide-tcp is a forensic tool that identifies TCP/UDP ports that are listening but are not listed in /bin/netstat through brute forcing of all TCP/UDP ports available.

To install it open the terminal and type the follwing command:

sudo apt-get install unhide

To scan run:

unhide-tcp

Why doesn't nmap show all open ports?

According to nmap.org:

By default, Nmap scans the most common 1,000 ports for each protocol.

You should use the unhide-tcp tool :

unhide-tcp is a forensic tool that identifies TCP/UDP ports that are listening but are not listed in /bin/netstat through brute forcing of all TCP/UDP ports available.

To install it open the terminal and type the follwing command:

sudo apt-get install unhide

To scan run:

unhide-tcp

Why doesn't nmap show all open ports?

The question states that -p- option is used. So it must have another reason.