Hi, I submitted mine on 5th January, still waiting for an update. I was able to successfully link my application in the account after confirming the application number with IRCC's helpline support. The application shows the date of submission as 17th January, coincidentally the same date that I was able to link it on.
I am not sure if there some glitch in the system that prohibits the linkage in first few days or there is actually a logical reason why it happens. Because initially when I did not have my application number, I was not able to retrieve too through UCI. Then one day I called IRCC to ask for support and my application number, tried after 10 minutes and it worked. But next time when you contact IRCC, confirm your city of birth on file (any spaces, commas etc), just to rule out any discrepancy.
Continue to post any updates here, I am sure it helps others too.