I have had several companies tell me so far that they never received applications I submitted via Stack Overflow Careers. Others have gone through successfully. One company told me that they had experienced the issue with a number of applicants.
Thanks for the examples! And apologies for the delay, I did not see your comment. I had a look, and as far as I can tell everything looks good. I see your applications for both jobs, and we sent notification emails to both employers (one on May 20th and one on August 17th). One of them went as far as to annotate your application. So while it may still be possible that something was up, it may also simply be that the companies did not remember your application.