There are a few reasons that can happen! First off, you can often fix this issue just by deleting the files that aren't opening and re-downloading them fresh.The problem could also be that the device you're using can't open .zip files without help. This usually happens on mobile devices like iPhones or tablets.You can get a free .zip opening app for Apple mobile devices here. Google has created a walkthrough for opening .zip files on Android mobile devices here.If you're still running into any issues, please don't hesitate to contact me.