This is a strange problem.
Few days ago I realised that recent tracks I listened to didn't scrobble to last.fm.
I tried to remove this problem, and didn't find any good way to solve it. So I went to last.fm help forum.
Where I found this thread. As you can see I'm not the only one having the problems with Amarok.
So if you read the thread, you know, that I thought I solved the problem, when I deleted the Amarok settings.
Yes my scrobbles magically appeared, but it still stops scrobbling after a while, and I have no idea why.
So next thing I did was to replicate the way user AntikristinA solved it, by also deleting Last.fm settings.
That'd didn't solve it either.
Also just disabling Last.fm scrobbling in Amarok, and once I enable it back, the songs I scrobbled before I disabled it get scrobbled.
So I really don't know if it's Amarok or Last.fm problem.

I checked with some tests yesterday and today, and it turns out that last.fm is handling it's library in a very strange way:

If I look at http://www.last.fm/user/balou59/library/recent only one track played yesterday shows, none of the ones listened to today. But if I check in http://www.last.fm/user/balou59/tracks all tracks are scrobbled in real time correctly.

The library is the first link shown when you log into last.fm, the second link shows if you check in your profile directly.

So apparently Amarok scrobbles correctly, but the library in last.fm is not updated.

Yeah I've just re-checked the thread on last.fm and apparently it's more wide-spread than I thought.
Thanks for help still.