Haven't used them, but I might have some thoughts.
Does it output pdfs that you print from acrobat? If so, are you printing fit to page?
Are you loading these in a bypass or manual feed, or are you using the main tray? I recommend using a bypass or manual feed if available and load a minimal number of sheets.
Are you rollers clean? Dusty rollers can cause slippage.
Sorry you are having trouble. Labels are expensive and it sucks to lose them due to printing errors.
Online labels has a similar layout for less BTW. They will send you free samples too.