I noticed in search.log that there are "INFO LookupOperator - Loading lookup table=..." log events that don't apply to the sourcetypes specified in the search. Later there is another event that says "INFO LookupOperator - Disabling automatic lookup of table=..."
Why is Splunk loading lookup tables that aren't used by any of the source types specified in the search?
I believe Lookups are available in any search, otherwise you would have to specify in every search to include a lookup. You can view more in the documentation related to lookups. Hope this answer helps clarifying your question.
P.S. Also check that your lookup field names are not repeated for another lookup table. That could certainly be an issue as well.
http://docs.splunk.com/Documentation/Splunk/7.1.2/Knowledge/Addfieldsfromexternaldatasources