Hi,
Thanks for using our plugins. I can help troubleshoot this with you.
I visited your site and I am seeing the January Link.
However, I had to directly go to the December Month View as the navigation is running our of memory for me. Can you increase you PHP memory on your server and that might help resolve that. If you are not sure about that your host might be able to help out.
Let me know if you go directly to December you see the link to and if you navigate between months you do not see it.
Add this to the month view url to go directly to December:
2015-12/
Let me know what you find out.
Thanks