If you're using asp.net you may have hit the json serialization size limit, happened to me. Edit your web.config file to set a larger size limit, it should be there already but commented out and looks like this
<jsonSerialization maxJsonLength="2000000" -- You received this message because you are subscribed to the Google Groups "Google Maps JavaScript API v3" group. To post to this group, send email to google-maps-js-api-v3@googlegroups.com. To unsubscribe from this group, send email to google-maps-js-api-v3+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/google-maps-js-api-v3?hl=en.