It sounds as though your document is not set-up correctly.

For table styles to work correctly the table must be inserted into a paragraph formatted with a style that matches the document defaults. This is because table styles can only override the document defaults, not paragraph styles.

You can check the document defaults by displaying the Styles pane and then clicking the Manage Styles button. The defaults are shown on the Set Defaults tab.

You then need to ensure that you have a paragraph style that matches the defaults. Typically, this would be the Normal style.

This has been the case since Office 2007.

Further reading on the hierarchy of Styles:

BEHIND THE CURTAIN: STYLES’ ORDER OF OPERATIONS
Word Blog - BEHIND THE CURTAIN: STYLES’ ORDER OF OPERATIONS

BEHIND THE CURTAIN: STYLES, DOC DEFAULTS, STYLE SETS, AND THEMES
Microsoft Word - BEHIND THE CURTAIN: STYLES, DOC DEFAULTS, STYLE SETS, AND THEMES