Start from scratch, meaning you'll redo the domain mappings (you won't have to reinstall WordPress). You have to install the plugin correctly, and then configure each site correctly.
You also can't have site1, site2, etc created as actual subdomains (via CPanel or whatever tool your hosting account uses).
I wrote detailed Domain Mapping instructions, including how to get things back to working state after trying "everything under the sun" to get it working.