Antminer S9 13.5Th/s is giving 9.5Th/s only
First of all - check your logs on the miner and then the status via the buildt in GUI on the 9S.

From what i can guess, one of your hashboards arent running. 
And there can be several reasons for this.

Reboot the miner,
Update Firmware,
Check PSU,
Check Netcable,
and Check Fans

IF non of this helped you need to look into the logs more closely,.
a board might be defective.