Problem with the Registration to a conversation

I got the following problem twice this week. Some people signed up for a conversation but it didn’t appear in my agenda so that I missed the conversation. It seems something is going wrong. Does anyone else have this kind of problem?

Hi Serge,
Sorry about that. I wasn’t able to find any of your conversation which you’ve missed. You’ve scheduled 2 conversation for today. One was already help and you’ve sent report for it, and second should be held in half an hour. You also have 2 conversations scheduled for tomorrow, do they appear at your agenda?