Existing text-based tables in a buffer, which lack the special text properties applied by M-x table-insert, are not treated specially as tables. To apply those text properties, type M-x table-recognize. This command scans the current buffer, recognizes valid table cells, and applies the relevant text properties. Conversely, type M-x table-unrecognize to unrecognize all tables in the current buffer, removing the special text properties and converting tables back to plain text.
You can also use the following commands to selectively recognize or unrecognize tables:
Recognize tables within the current region.
Unrecognize tables within the current region.
Recognize the table at point and activate it.
Deactivate the table at point.
Recognize the cell at point and activate it.
Deactivate the cell at point.
See Converting Between Plain Text and Tables, for another way to recognize a table.