What program are you using to align the labels?
Assuming they are Avery brand labels, it is best to use the Avery software either the Avery Wizard (free) or Avery Design Pro (the Lite version is free).
You can get these programs from the Avery web site.
Also, check the paper feed rollers in the printer. If the printer has been well used, it could be that the rollers have worn smooth. In this case, roughen up the surface of the rollers with the eraser that sits on the top of many pencils.