It could be a few different things: And these are all just guesses from experience:
- The application is running slow because of some looping process that is taking all the process power
- The field you are using might be mixed up with a different/older/new field that doesn't have the data in it. (this often happens when people override the field names on forms or reports).
- The data set you are looking doesn't have any matches.
- The report has some other filter that is causing it to return null.
- You table is too big....
Again, just some ideas/guesses