You say it hangs. What exactly does that mean? What do you see? OCR is a time consuming task and it sometimes takes a bit longer to get your job processed. Leave the status window open and wait until processing finishes. Sometimes our OCR backend is a bit busy. All jobs are queued and we process them in order and this could take a couple of minutes. We know that there are some peaks and we will increase our worker machines in the near future.
You should use the PDF24 Creator if you can. This is a software for Windows and this one can create PDF/A files. Our online OCR tool is sometimes a bit busy in peak cases. We increase our workers periodically, but sometimes it comes to peaks and a lot of people use our services and then it can take time to get a result. My advise is, use the PDF24 Creator if you can.