A missing bottom row may happen with some label/printer combinations. That's due to the fact that many labels have 0.5" between the bottom of the last label and the trailing edge of the label sheet while the printer's maximum printable area leaves a bottom margin of just a bit more than 0.5". Filemaker then sees that the fifth row of labels will extend out of the printer's printable area and generates a page break to insure that no part of the bottom row of labels will be left unprinted.
You can usually "tweak" your layout part sizes by very small amounts to get your labels to print correctly for such a printer. Try reducing the size of the header 1 pixel at a time, previewing the result with the correct printer settings specified in Printer setup and see if that works for you.
To change a layout part size this way, bring up the object info pallete, click the units shown repeatedly until pixels are shown, then click the layout's part label. Now you can modify the part's height and the part height will change to the new size when you click into another box in the palette.