Logstash '_grokparsefailure' issue

When i added the add_tag statement.

Kibana was showing the correct log for a few seconds

then again, it was displaying somwthing as

host:localhost.localdomain tags:beats_input_codec_plain_applied, _grokparsefailure _id:AVSfyfAcoQty0p9BNk2p _type:MyLog _index:filebeat-2016.05.11 _score: -

Filebeat debug has something like this,

2016/05/11 12:23:32.087717 output.go:87: DBG  output worker: publish 1808 events
2016/05/11 12:23:32.087761 client.go:146: DBG  Try to publish 1808 events to logstash with window size 1024

is it due to the heavy log count.
I have already increased congestion_threshold => 200

What can i do to fix it.