It's currently a known issue due to the update accidentally breaking some of these tasks.
Here's the main thread for the issue: https://answers.ea.com/t5/The-Simpsons-Tapped-Out/Under-Investigation-Unable-to-Progress-Storylines/td-p/10748925/jump-to/first-unread-message
Unfortunately it doesn't look like it will be fixed until the new event rolls around and they become available again on their own so just keep playing in the meantime by sending characters onto what tasks you do have to build up $$ and XP. It will potentially take anywhere from a couple of days to a couple of weeks...
