Due to a bug, some users may have noticed that your fantasy leagues are shown as successfully synced but no players were added to tracking. This has been fixed as of 9:30 PM EST on the 15th of June. You do not have to do anything to fix the problem. Last night all of your leagues were updated.
However, keep in mind that free accounts are limited to tracking up to 50 players. If synced a lot of teams then not all of your fantasy players will be added to player tracking. If you want to make sure that one particular team’s players will be synced, make sure you mark that fantasy league as your favorite.