For each row-level failure, the crawl log contains details on the reason for the failure. The error counts are summarized at the end of a full or incremental population.
There are other failures that can impact the indexing process itself and prevent the population from completing:
* The full-text index exceeds the limit for the number of rows that can be contained in a full-text catalog.
* A clustered index or full-text key index on the table being indexed gets altered, dropped, or rebuilt.
* A hardware failure or disk corruption results in the corruption of the full-text catalog.
* A file group that contains the table being full-text indexed goes offline, or is made read-only.
You should view the crawl log at the end of any significant full-text index population operation, or when you find that a population did not complete.
Refer the full text index articles at: SQL Full Text Search Programming