Why is full text index falling behind?
There seems to be a few queries you can try to get more information out of what is happening. The below query that was pulled from Stackoverflow takes advantage of FULLTEXTCATALOGPROPERTY.

DECLARE @CatalogName VARCHAR(MAX)
SET     @CatalogName = 'FTS_Demo_Catalog'

SELECT
    DATEADD(ss, FULLTEXTCATALOGPROPERTY(@CatalogName,'PopulateCompletionAge'), '1/1/1990') AS LastPopulated
    ,(SELECT CASE FULLTEXTCATALOGPROPERTY(@CatalogName,'PopulateStatus')
        WHEN 0 THEN 'Idle'
        WHEN 1 THEN 'Full Population In Progress'
        WHEN 2 THEN 'Paused'
        WHEN 3 THEN 'Throttled'
        WHEN 4 THEN 'Recovering'
        WHEN 5 THEN 'Shutdown'
        WHEN 6 THEN 'Incremental Population In Progress'
        WHEN 7 THEN 'Building Index'
        WHEN 8 THEN 'Disk Full.  Paused'
        WHEN 9 THEN 'Change Tracking' END) AS PopulateStatus
FROM sys.fulltext_catalogs AS cat


However, In this article, microsoft mentions 


  it is usually a better option to check the corresponding
  PopulateStatus property at the table level,
  TableFullTextPopulateStatus in the OBJECTPROPERTYEX system function.
  This and other new full-text properties in OBJECTPROPERTYEX provide
  more granular information about full-text indexing tables.


To this, I would use the following script:

DECLARE @TableId VARCHAR(MAX)
SET @TableId = OBJECT_ID('gGastroversion')

Select CASE OBJECTPROPERTYEX ( @TableId , 'TableFullTextPopulateStatus' )
            WHEN 0 THEN 'Idle'
            WHEN 1 THEN 'Full Population In Progress'
            WHEN 2 THEN 'Full population is in progress.'
            WHEN 3 THEN 'Propagation of tracked changes is in progress.'
            WHEN 4 THEN 'Background update index is in progress, such as autochange tracking.'
            WHEN 5 THEN 'Full-text indexing is throttled or paused.'
            WHEN 6 THEN 'An error has occurred. Examine the crawl log for details'
        END


Notice, that if you have number 6 as a result it will tell you to check the crawl log. Troubleshooting this information can be found here.