Well, I think it says on the paid click page that they are sent out in batches and remain until they've all been clicked on. So maybe sometimes you are just missing them. Sometimes, too, they are sent out to people with certain demographics, so there's another reason. Sometimes I have seen people post about getting paid clicks, and they aren't there for me, either. Some day I don't get any, and some days I do!
In some cases, I think, yes, your earning history probably has something to do with it as to whether a certain paid click is available to you or not.
You've just got to keep checking often - and soon enough you'll find one for you!