Queries :: Multi-field Query Not Working When Some Fields Are Null Value

Access 2010. I've had to learn it at work because our DBA was let go and I was the only one willing to give it a go. Read a book or two and picked up some stuff on the internet.

Here's my problem:
I have a simple table -Employee with 4 fields. FirstName, LastName, Office and JobTitle. I have form called Form1 that has 3 control fields cboJobTitle (a combo box that is populated by a query that finds all the unique values of that field in the Employee table), cboOffice (same as above) and txtName (a text box to allow user input) that are used as the criteria for a multi-field query triggered by a button at the bottom of the form. The idea being that you could do a search using this form to find all the employees in one office or all the accountants in one office, or any other combination.

The main search query has the following criteria for each field -
Like [forms]![Form1]![cboOffice] & "*"
Like [forms]![Form1]![cboJobTitle] & "*"
Like [forms]![Form1]![txtName] & "*"

It works great...until I enter a record where one of those fields may be null, such as if I leave the JobTitle blank.
If I have two employees in an office in one city and then do a search for all the employees in that office, it only returns one record and ignores the one that has the null value in the JobTitle field.