Hi!
First, make sure the index table is properly configured, saved and created. Try a configuration based on these steps, then create a new index: http://i.imgur.com/cB8BWBg.png
Then, make sure that the search instance configuration is correct as well: http://i.imgur.com/NAnLvdo.png
Remember, that the index table can be used by multiple search instances with different configuration.
Hopefully it’s just something with the configuration.