I logged this under iOS, but it happens on the desktop too.
It seems like last.fm integration just dies after some time. On my laptop, it sometimes would just stop scrobbling. This happens a lot when I leave my laptop on (and spotify running), and use spotify on another device throughout the day, but then come back home and play my music there. It just stops scrobbling.
On my iOS device, when I wake up and play spotify, it won't scrobble. I have to force quit the app and start it so it would. This happens throughout the day as well. It happens when I stop playing for a few hours and bring back spotify from being idle.
I don't know why that is the case. Sometimes at home my internet connection dies, sometimes on my phone my LTE/data connection is lost and I understand that it could cut connection to last.fm (not that it needs it 100% of the time) but if spotify can get online and play tracks, it should be able to get connected to last.fm and scrobble.
Is anyone else experiencing the same issue?