Logs from grayLog is getting deleted

Hi,
I have done the grayLog2 setup for my prod server , with GELF UDP input type. From last few weeks the logs are getting overwritten/deleted every sec.
I have tried creating new index, but unable to set it as default index. I am unable to figure out the root cause of this issue, it would be great if anyone can help.
Thanks in advance.

whats your retention policy under indices ?