I just got the new batch of business cards that we ordered: they don’t match the batches we ordered before. Did something get changed in the artwork? How do we fix this?

No, the color values on the card artwork are all the same as in prior (successful) batches of printing. I haven’t changed a thing having to do with the color.
That’s one of the drawbacks of digital printing. While you can get relatively low quantities of cards at lower prices than traditional printing, you don’t get the consistency of color that you would with traditional printing – especially when the batches are printed at different times. The calibration of the printer can change, or the way that they process the files can change, and you can wind up with different colors even though the artwork hasn’t changed.
So, every time we send them something to print, we’re hoping that their results will be consistent – but that’s not always the case.
If getting the right color is really that important, then it might make sense to look into traditional printing – that guarantees that the colors will always be consistent. Traditional printers use Pantone inks, which have greater color accuracy than the CMYK process used by digital printers.