Can’t get add-ons to work!

Hi. I am using Kodi 17 on a Windows 10 machine and also on an Nvidia Shield. On neither platform can I get add-ons to install from a zip file, despite following step-by-step guides to the letter. The problem must, I suppose, be pilot error, but I cannot see where I could possibly be going wrong. I go to File Manager, Add Source, then type in the URL and assign a ‘friendly name’. Then back to Add-Ons, click the ‘box’ icon, Install From Zip File, enter the given name, make the necessary choices, Install From Repository, make selection, and I see a box telling me the add-on has been added. Then, back to Video Add-Ons: nothing! No video add-ons available. Zero. I have watched several YouTube tutorials etc and followed exactly, but I simply cannot get any add-ons to appear. Any suggestions would be gratefully received.