Are you using an outdated import filter? The Endnote website shows the most recent OCLC Worldcat import filter is dated 2013-09-05: http://endnote.com/downloads/filters?field_filter_data_provider_value_op=starts&field_filter_data_provider_value=oclc&=Apply
If you’re using a current import filter then the issue may be a mismatch between the exported field data and the Endnote import filter’s tags. If this is the case it’s possible to modify the filter if provided a sample of the exported records and data fields in order to compare them against the Endnote import filter.