Start by installing the latest version of Zotero, 6.0.26, from the download page. (You should also figure out why you haven't received automatic updates in months.)
If you're still having trouble after that, provide a Debug ID for an import attempt that fails.
@GinaMontenegro: It looks like a recent change broke importing in earlier Zotero 6 versions. We've pushed out a fix, so your Zotero should start working again, but you should still update to the current version of Zotero when you can. We can only ever provide support for the current version.
If you're still having trouble after that, provide a Debug ID for an import attempt that fails.