When you create labels using the Label Wizard -and- you choose "Across, then Down" for the Column Layout, you will receive a message like this:
Some data may not be displayed.
There is not enough horizontal space on the page for the number of columns and column spacing you specified.
Click Page Setup on the File menu, click the Columns tab, and then reduce the number of columns or the size of the columns.
...funny thing is, Access is 100% correct - the Wizard makes the layout too wide to fit; imagine that. Even funnier is the fact that if you select "Down, then Across" the message goes away even though the fact remain that the layout really is too wide for the page.
However, the suggestion from the Access message doesn't really help - what you have to do is reduce the column widths -but- increase the column spacing. In that way, you won't alter the horizontal alignment of the printing.
In my example, using Avery 5160/5260 labels which are 3x10 per sheet, Access set the left/right margins to 0.25", columns to 2.625" and the spacing to 0.125" which totals 8.625" which is obviously more than 8.5" for standard