Do I understand correctly that the lookup table is an Excel sheet derived form a text file?
I assume the real lookup you are preforming and having problems with is for another file. Have you tried testing the lookup on the originating file?
If the excel lookup does not work using its souorce file there is clearly some sort of anomaly either in the creation of the Excel table or in the lookup process.
If the lookup works with the source file but not the intended file for which it was created then it may be that there is a difficult to spot data problem.
Does this provide you with anything new to consider or have I just covered old ground?
This may be over simplistic but I tend to find when I have a problem with an external lookup it is due to looking up a 'Named Range' vs. a 'Worksheet'. As your lookup file is added to the named range no longer covers all possibilities leading to the problem you mentioned.
Learned from plenty of head scratching in the past!
Hope this helps.