Graham Mayor posted the following list of common problems a while back:
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There are a number of possibilities -

1.  A header/footer in normal.dot. Check by running Word in its safe mode
(hold the CTRL key whilst starting).
2. Check that there is no scaling option set in the file > print dialog.
3. Check that the paper size in the print driver and page settings match.
4. Some printers are poor with label handling allowing slippage as the paper
passes through the print path. Clean the rollers.
5. HP has historically produced some lousy drivers - see if there is an
update for yours at http://www.hp.com/pond/ljbeta/

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As far as (2) is concerned, I would add "uncheck the Allow A4/Letter paper
resizing option"

However, if you produce an ordinary 1-page document filled with text with
known margins (say 1in or 2.5cm) and print it, you can then measure the
margins as printed. If they are significantly different then you will
obviously have trouble printing labels. Personally, I would want a printer
that did what it was told, but if that's not an option, for the labels the
best you can do is to adjust the page margins in your Mail Merge Main
Document (File|Page Setup) until the labels print OK. You can usually use a
single sheet of ordinary paper multiple times to avoid waste (e.g. change
the text colour each time, and place the paper over the label sheet to check
alignment).

Peter Jamieson