apt-get update not fetching packages in external repositories

[Hariharan Iyer]

apt-get update is not fetching packages from external repositories in Xenial. I have a few added in my system, but let's consider the following two as examples:
http://ppa.launchpad.net/noobslab/icons/ubuntu
This provides a package called ultra-flat-icons, among many others

http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu
This provides a package called firefox-next

Neither of the above-mentioned packages show up in apt-cache search after an update:
~ » sudo apt-get update
Hit:1 http://ppa.launchpad.net/mozillateam/firefox-next/ubuntu xenial InRelease
Ign:2 http://repo.vivaldi.com/beta/deb stable InRelease
Hit:3 http://ppa.launchpad.net/noobslab/icons/ubuntu xenial InRelease
Get:4 http://archive.ubuntu.com/ubuntu xenial InRelease [95.8 kB]
Hit:5 http://repo.vivaldi.com/beta/deb stable Release
Hit:7 http://archive.canonical.com/ubuntu xenial InRelease
Hit:8 https://deb.opera.com/opera-developer stable InRelease
Hit:9 http://archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:10 http://archive.ubuntu.com/ubuntu xenial-backports InRelease
Hit:11 http://archive.ubuntu.com/ubuntu xenial-security InRelease
Get:12 http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages [7,326 kB]
Get:13 http://archive.ubuntu.com/ubuntu xenial/universe i386 Packages [7,307 kB]
Get:14 http://archive.ubuntu.com/ubuntu xenial/multiverse amd64 Packages [139 kB]
Get:15 http://archive.ubuntu.com/ubuntu xenial/multiverse i386 Packages [136 kB]
Fetched 15.0 MB in 25s (600 kB/s)
Reading package lists... Done
~ » apt-cache search firefox | grep next
~ » apt-cache search firefox | grep beta
~ » apt-cache search ultra-flat

Version information:
 » lsb_release -rd
Description: Ubuntu Xenial Xerus (development branch)
Release: 16.04

~ » apt-cache policy apt
apt:
  Installed: 1.2.4
  Candidate: 1.2.4
  Version table:
 *** 1.2.4 500
        500 http://archive.ubuntu.com/ubuntu xenial/main amd64 Packages
        100 /var/lib/dpkg/status

[Julian Andres Klode (juliank)]

Neither repository provides the packages you mentioned for xenial. The first only provides humanitary-icon-theme, the other no firefox-next, only firefox, firefox-dev, and some locale and debugging packages.