Ah, that happened to me, too. Just take them all out and race every single one. The trophy will pop up eventually.

Oooh a cubi race! :-) I'll try that.

No, you need to have all at the same time in your inventory.

...at least for the S1-Collector-Trophy.

@Herr Jim: It could possibly be both (and a bug, while we're at it). I remember knowing way back in the day that the trophy didn't pop up immediately, so I went and put all of them into my inventory -- but, nothing happened. So, I asked on Global, and somebody asked if I had raced all of them. I knew I hadn't, so I went down the line. Six or seven in, the trophy popped up. So, I do think that it's somehow different for each person -- but how that works, I have no idea.

Never heard of that, interesting. Thanks.
For my S2-Trophy, i know for sure, that i hadn´t raced all.