Two possibilities:
1. You have a page footer that's bigger than 1/2 inch. The bottom margin, the page footer and a little-known parameter called the From Edge distance are interrelated. The page footer always starts at the From Edge distance (1/2 inch by default), and, if big enough, can push the apparent bottom margin beyond the real bottom margin. You can check the From Edge distance this way:
In versions of Windows prior to Word 2007:
1. Click the File->Page Setup menu item.
2. Click the Layout tab.
In Word 2007, 2010:
1. Click the Page Layout tab on the Ribbon.
2. Click the little arrow in the lower right corner of the Page Setup group.
3. Continue as in Step 2, above.
The other possibility, which won't affect every page, is that you have Widow and Orphan Control activated. A widow is the last line of a paragraph alone at the top of a page. An orphan is the reverse: a single line on the first page followed by the rest of the paragraph on the next.
Typesetters apparently consider those isolated lines uglier than a chunk of blank space at the bottom of a page. Go figure.
In Word versions prior to Word 2007:
1. Click in an affected paragraph.
2. Click on the Format->Styles and Formatting menu item.
3. In the bottom part of the sidebar that pops up, the style for your paragraph should have a bold box around it.
4. Hover the cursor over the style. A listbox arrow should appear. Click it.
5. Select Modify from the list box.
6. Click the Format button and select Paragraph.
7. On the Line and Page Breaks tab, clear the Widow/Orphan Control checkbox.
8. OK out of everything.
All paragraphs in the document with that style should adjust themselves.
Now, if you want to carry this one step further and fix the style on the template your document is based on (probably Normal.dot, but not necessarily), do this:
1. Click the Tools->Organizer menu item.
2. In the left-hand list box, find and click on the style you changed.
3. Click the Copy button between the two list boxes.
4. Click OK.
When you close your document, you may see a warning message asking if you want to save the changes to Normal.dot. This is an anti-virus measure, intended to alert you that something has changed in Normal.dot. In this case, you made the change, so you want to accept it.
In Word 2007, possibly 2010:
1. Click in an affected paragraph.
2. Click the Home tab on the Ribbon.
3. Click the arrow in the lower right corner of the Styles section.
4. The style for your paragraph should have a bold box around it. You may have to scroll down to find it.
5. Hover the cursor over the style. A listbox arrow should appear. Click it.
6. Select Modify from the list box.
7. Click the Format button and select Paragraph.
8. On the Line and Page Breaks tab, clear the Widow/Orphan Control checkbox.
9. Click OK.
10. If you want this to carry forward to other documents based on the template this one is based on, click the radio button for New Documents Based On This Template.
11. OK out of everything.
All paragraphs in the document with that style should adjust themselves.
When you close your document, you may see a warning message asking if you want to save the changes to Normal.dotx. This is an anti-virus measure, intended to alert you that something has changed in Normal.dotx. In this case, you made the change, so you want to accept it.
Hope that helps.