By far the most common issue with CBS is a notice attached to the CBS league.
Sometimes, when a commissioner needs to take a certain action, CBS blocks all access - including to us importing the league - until a notice on the CBS website is addressed.
If a CBS league fails to import, please check the league on the CBS website and make sure there is not a notice that needs your attention blocking your access to the league.
Also, please note that 2020-2021 CBS Fantasy Leagues won't be accessible until CBS opens up their 2020 - 2021 fantasy leagues. That is often as late as June in any given year.