Yes, I have that happen to. Sometimes it works on the first try and sometimes it takes a number of attempts. It could be coincidence but I find it I try to drop the icon over the date (like 25) it works more often. But sometimes that doesn't work either. The next best think is to go out to a different page (like rotations or something) and then back in and it works. I actually have deleted and readded a couple of times because that was faster.
Jo