Hi everyone,
I've figured out the problem. I was using the iZip app, which seemed to be the problem. I wasn't aware that Mac OS (El Capitan, at least) has its own compression app, called 'Archive Utility'. I deleted iZip from the App folder (viewed in finder), right-clicked the .zip file and selected 'open with' > 'Archive Utility,' problem solved. Hope this helps any other unsuspecting victims such as me! Thanks to all those who tried to help. Best wishes.