I have had limited success with the other suggestions - basically, Word is buggy in this area. There is a simple alternative - indeed the only alternative AKAIK if you want the hyperlink to vary AND the text that is displayed to vary too (not an unreasonable requirement).
Note that this only works for DOCUMENT MERGES, not for EMAIL MERGES, since it relies on processesing the output document.
The trick is to add a place marker (text that serves as an ID) wherever you want a hyperlink. Insert this via a regular mergefield. In your table of source data, you need columns
place marker, text_for_display, hyperlink
You then need to put your source data into Excel (if that isn't where you have it already) and put this formula
="Set Rng = ActiveDocument.Content: Rng.Find.Execute FindText:="""&[place marker]&""": ActiveDocument.Hyperlinks.Add Anchor:=Rng, Address:="""&[hyperlink]&""", TextToDisplay:="""&[text_for_display]&""""
into a blank column in the first row, and fill down.
You need to amend [place marker],[hyperlink] and [text_for_display] to the appropriate cell references.
If the [hyperlink] includes a query string, you may find you want to build it using a formula, based on other data in the source.
The resulting formulae can then be pasted into a macro and run on the output document.
You may then want to use the "Robbins/Mayor" macro http://www.wordbanter.com/showthread.php?t=18346 to split the generated documents.