Help - Error : Could not connect to master of shard replica set

I have a shard, using mongo v2.4.11. It has to mongos, 3 config, and 8 replica set (2 replica for each set, means 16 node for shard)

It is keep having exception as per below.

Caused by: com.mongodb.MongoException: can't connect to new replica set master [shardA01:27017], err: couldn't connect to server shardA01:27017

I try to look for anyone who might has similar issue, but i found nothing. This happens about 1 of 1000 request.

All node of shard is hosted using aws virtual servers.

What i have done is

- check CPU and memory but everything is fine, not even > 10% of usage

- set master priority to one of the node.

Anyone know what the problem is? Thank you so much.