Play count has dissapeared from some songs?

Basically, I've found that recently the play count from some songs on my itunes has completely dissapeared, even though itunes still shows the date that the song was last played. So in summary my itunes shows when I last played some songs but not how many times I've listened to it, surely they should be on at least 1.

Has anyone had this problem before? Does anybody know how to fix it?

Cheers.

P.S - I've inserted a print screen below to show what I mean. (As you can see on the right, these songs have the data of when they were last played, but not the play count)