can you double check your lookup table name? if your not careful you can forget to add ".csv" at the end of your lookup file upload process. easiest way to check is to go to where you added the lookup table in splunk lookup table manager and look at the name, does it end with ".csv"? that is the first thing that comes to mind.
if your lookup table does have a ".csv" ending, then you need to double check your permissions. the lookup table and definition need to have global permissions.