The ESV translation is the only translation (as far as I have seen) that has published an API for programmers, so that they can easily interact with the text. For this reason and that it is my own translation of choice has lead to the decision for the service to be offered in the ESV translation.
The front and back sides of the cards are contained in the same PDF. Begin by printing the first half of the pages, then reload the paper in the correct order and orientation and finally print the last half of the pages.
The margins of the PDF are already centered properly for printing, therefore the problem lies with your printer. You may manually adjust the margins using the margin offset feature or you could try re-aligning your printer.
The problem could be an issue on your end, my end, or the ESV Bible Web Service's end. I can't say for sure what caused it, but my suggestion would simply to try again later.
Another possibility is that the query quota has been reached. The ESV Bible Web Service only allows 5,000 queries a day from a single server; therefore it is possible that this limit may have been reached. This issue can be avoided if anyone would like to purchase an expanded-access key. (Additional Info)
A PDF file requires Adobe Reader to view. Adobe Reader can be obtained for free from Adobe's website.
I (Michael Scott) can be contacted by email at mcscott@mcscott.org. Please feel free to send any comments, suggestions, or bugs about the Verse Card Maker.