This may be something you're aware of, but seems like a lot of folks aren't. Having USB debugging mode is a must, but also is having the USB mode set correctly. I've tried using an older Nexus 7 (first version with Android 5.1.1 installed) and although it lags a bit will connect and work once all the settings are correct. You MUST be in USB PTP mode. Most android devices default to MPT mode initially. Some may automatically switch but the Nexus 7 won't. If you don't know how to change to PTP (camera) mode, on a Nexus 7, Settings > Storage > select the icon in top-right (3 dots) > USB computer connection. You should see an option for PTP or MTP. I seriously doubt it's the device, cable or other issue. You can manually turn on the USB debugging mode. Double check that it is on. If prompted by a pop up window after debugging mode is set, approve it's use and select don't ask again. Then set it to USB PTP mode. Then if it doesn't work after you a sure all these setting are correct, then troubleshoot for other issues.
To check to see if USB debugging is on, go to settings > system, you should see "developer options" if you do, select that and about 6 items down should be USB debugging. It should be on (slide green & to the right). If you do not have developer options, select "about tablet" and press "build number" 10 times to get it in USB debugging/developer mode on. Then go back & look in "developer options and ensure it's on and then exit and set to USB PTP mode.