Elasticsearch read timeout, seems to have too many shards?

I'm using both elasticsearch 1.4.4/2.1.0 with cluster of 5 hosts on AWS. In the config file, I've set index shard num to 10.
So here comes the strange behavior: I created a index everyday, and when there's about 400 shards or more, the whole cluster returns Read Timeout when using Buik insert API. If I delete some indices, the timeout error disappeared. Anyone meets similar problem? This is really a big obstacle for storing more data

How much data in the indices?

What type of instances are you using? How much heap have you allocated to Elasticsearch? Do you see anything in the Elasticsearch logs?