If you don't find any errors in your logs, SMS Inventory is probably working OK.
Now, *.TXT files are not part of the normal software inventory. You must have "told" SMS to specifically look for TXT files (By default, I beleive SMS only looks for EXE files.) Are you sure something didn't get messed up when you modified this?
Also, assuming that the inventory process is working OK, maybe your query is set up wrong? I would set a very OPEN query and make sure you get at least the EXE's back.
Let me know if this helps any... Joseph L. Poandl
MCSE 2000