Penguin Publishing has mandated that all library Kindle books from its company must be transferred via USB. If you go to check out a Penguin book, each page will tell you that you will be required to use a USB cable rather than a wireless transfer. At the Amazon site, you will again be guided to transfer via USB.
This would certainly not be our choice but, for right now, there is nothing we can do about it. Please contact Penguin if you wish to express your opinion about this change. But please let us know what you are experiencing so we can continue to help you and be advocates for you.