This sometimes happens to me when I am relisting an older item. There is no rhyme or reason. A couple of weeks ago, I waited a few hours and then went ahead and tried again...magic it relisted.
You can create a new ad and just copy and paste the verbiage from the old one to the new one. PITA but it works.
I also suggest that you check your active listings to see if it posted anyway. This happens to me often when the item doesn't post in time but anything is possible.