Because on Sundays it updates the entire database so sometimes you have to wait.
I've had it happen a few times when it wont update the songs I've listened to for close to 2 weeks. But don't worry, it still keeps a database on your computer and when it manages to connect you will update.
If you unplug your internet access you can still update the songs when you connect again. I unplugged my laptop for 3 weeks and the moment I plugged it back in it updated Last.FM with every song I listened to in that 3 week period.
I've had it happen a few times when it wont update the songs I've listened to for close to 2 weeks. But don't worry, it still keeps a database on your computer and when it manages to connect you will update.
If you unplug your internet access you can still update the songs when you connect again. I unplugged my laptop for 3 weeks and the moment I plugged it back in it updated Last.FM with every song I listened to in that 3 week period.