strange behaviour

Hello, as I cannot make real test on this, I’m not sure at all if it’s a “bug”. But I prefer to talk about it here.
I have a lot of unscheduled drafts and if I can move the fist ones into the calendar, the last ones cannot be moved. The 4 arrows over mouse cursor does not show – and it works for the same first posts.

http://ww.wp.xz.cn/extend/plugins/editorial-calendar/