Buying Books For My Kindle For PC Not Working

I'm not sure why but it seems that I cannot buy Kindle books for my Kindle for PC any longer. I bought several ebooks this way about a week ago, but now I cannot. I've checked everything I can think of, even reinstalled my Kindle for PC a couple of times, but nothing I do or check seems to do anything. I hope I can figure out this problem, I'd like to keep buying Kindle ebooks.