Exchange 2010 SP2 RU3 - Indexing backlog reached a critical limit of 48 hours

Hi

I've got a large deployment of Exchange 2010 SP2 RU3 with a single DAG configuration (in a single site).
Every few days I'm getting critical errors in the application log as follows:

event 5611
Source: CI Troubleshooter
Indexing backlog reached a critical limit of 48 hours or the number of items in the retry queue is greater than 10000 for one or more databases: DB57 (70, 57934, 0)

event 5604
Source: CI Troubleshooter
Indexing backlog reached a critical limit of 48 hours or the number of items in the retry queue is greater than 10000 for database: DB57

The databases regarded in the errors change (not consistantly the same database).

I ran:    Troubleshoot-CI.ps1 -database DB57 -sympton all

The result is always: Is DeadLocked : False

I don't understand the meaning of those errors, how can I monitor this queue, what may be the cause for this backlog and how do I fix the issue.
I can always rebuild the index catalog for the database, but is it the best or required way to handle this ?

Thanks