Transactional replication - undistributed commands

Hi

We have a problem in Transactional replication.

The problem is that I have mismatch number of rows between the publisher and the subscription.  

We have Transactional replication in SQL2012 Enterprise edition SP3

Publication – Standalone server

Subscription – AlwaysOn server (Push)

Distribution – 3^rd server Standalone

The Publisher/Subscriber combination have several publication that all work fine except 1.

The problematic publication has one table.

The table hasa column with XML  Data type, size – 260GB, 5.5 Millions rows.

The data  is notbeing replicated to the subscriber,

Undistributed commands show 919382 and counting

I think the problem probably in the distribution server.

The repl process try to exec  sys.sp_MSget_repl_commands;1 and wait for PREEMPTIVE_OS_WAITFORSINGLEOBJEC. I found that by using sp_whoisactive.

We get error message in distribution job:

Raised events that occur when an agent’s reader thread waits longer than the agent's -messageinterval time. (By default, the time is 60 seconds.) If you notice State 2 events that are recorded for an agent, this indicates that the agent is taking a long time to write changes to the destination.

In addition thereare  no database locks  

Please advise how  to solve the problem

Thanks & Regards

Erez Hasman