1) Try manually checking the data
| inputlookup mylookup.csv
that should tell you if it's there.
2) Try manually testing a lookup. Assuming the name of the lookup field is foo, and one record in the lookup the field foo has a value of "bar", and there is another column named baz.
| makeresults
| eval foo = "bar"
| lookup mylookup.csv foo OUTPUT baz
If both of those work, then you have probably set up the lookup itself incorrectly. If one of them fails, then either the definition is wrong, the lookup name is wrong, or the data is wrong.
