If you have added a Table type question in your Form, you can specify the dimensions of this table in the "Rows" and "Columns" fields of the Question Editor (in the Form Editor) - for example, a 2x2 table.
If the table does not display fully or properly in the Form preview, you may not have enough questions to properly fill the table - as in the 2x2 example, you would need 4 questions to fill the table. In contrast, if your questions aren't included in your table, you can make the dimensions larger to fit all the questions into the table.
In the Form Editor, you must order your questions so that they fall within the table:
Since our table is a 2x2 grid, questions #2-5 fit within that table (as indicated by the table icon beside the questions). Question #6 does not, since the table is already filled.