Trouble shooting is often one of the most difficult things to do.  The code above looks correct but its difficult to tell if it's accurate because we cannot see your fields and the underlying query.

First thing to do is check spelling.

Whenever I have an issue I make a copy to play with.  Strip out all but one field, then get that one field working.
Example:  If you put in the query criteria "IS NULL" does it bring back all of the null records?  If that worked, add in a hard coded value that you know should work. If one of your SubCategory values is "XXX" then make it
           Like *X* OR IS NULL
Did that work? If that works the substitute in the variable from the form.
               Like "*" & [Forms]![frmSearchList]![txtSubCategory] & "*" Or Is Null
If that gives you the results you are looking for move on to the next field.  Testing each time.

What you think you are asking of Access is not always what you are actually asking of Access.

Just be methodical testing one thing at a time.  When you change multiple things and it's broken you don't know which one thing broke it.

The advantage of the Gold membership is that you can download the sample database and compare to yours.
You can also take advantage of the fact that you can watch and re-watch over and over the videos until the material sinks in.  (For me that can be 3 or 4 times if not more.  I often pick up little tidbits I missed the firs or second time through.)

Yeah, that Search 2.0 is about as simple as I can make that process without programming. Of course, I show a much better method that involves a significant amount of VBA and SQL in the Search Seminar.