It has step-by-step guide on interrogating html table (Excercise 1 - Task 1). If it doesn't help, please provide screenshot of your table, screenshot of HTML Table Designer window with created schema and controls and more details on what happens then number of rows is changed.
It looks like not all of your matching elements are matching. If your columns headings change you can modify the match settings for them. Once you have a match for your table section your cells should match. if a column heading is not always there - I would start be setting it to be optional.