The range lookup seaches must be sorted (ascending). If the lookup function does not find the value to be looked up, it will return the value just before it.
It sounds to me like the value you are looking for and the value in your lookup range might be just a bit different (maybe a trailing space?).
Whenever I do lookups, I like to nest them:
=IF(LOOKUP(C4,F4:F14,F4:F14)=C4,LOOKUP(C4,F4:F15,G4:G14),"Not Found")
Where C4 is the value to search for, F4:F14 is the search range and G4:G14 is the return range.
Hope this helps,
K