Ubuntu doesn't recognize Android devices anymore

I have problems with my Ubuntu GNOME 14.04 installation recognizing my Nexus 7 at times, too. This is what works for me:

1. Make sure the cable can transfer information, that it's not just a power cable.
2. Make sure you have USB debugging enabled.
3. Unlock your device.
4. Connect your device to your computer.
5. Your device should display a pop-up, asking if you want to accept a connection to your computer. (If you had this pop-up before, you might have ticked the "always accept a connection from this device" checkbox.)
6. Normally, your device should now pop up in your favorite file manager. If it doesn't, check if ADB finds your device. In a terminal, type adb devices (you might need to be root).
7. If ADB lists your device, go to Settings > Storage and select the options button in the upper right corner:

8. Select the file sharing option you want:

The problem was ultimately the cable.

Some USB cables are just for charging. Other cables also support data. I tried 3-4 different cables, until one of them ultimately worked.

That was the reason, why connecting the phone didn't even register with dmesg at all.

To set up device detection on Ubuntu Linux:

Log in as root and create this file: /etc/udev/rules.d/51-android.rules.

Use this format to add each vendor to the file:

SUBSYSTEM=="usb", ATTR{idVendor}=="0bb4", MODE="0666", GROUP="plugdev"

In this example, the vendor ID is for HTC. The MODE assignment specifies read/write permissions, and GROUP defines which Unix group owns the device node.

Now execute:

 sudo chmod a+r /etc/udev/rules.d/51-android.rules

From developer.android.com/studio/run/device.html