Data Loss

I recently bought a new phone. While I was trying to open OneCalendar with my new phone, although I entered my account, the previous dates I planned two months ago didn't show up. I can not see what I have done for the last two months. There is an interesting situation. When I re-download OneCalendar app in my old phone and enter my account I can see the period of the last two months. As far as I am concerned data is stored in the cloud so, what might be the problem. If I can not find a solution ı have to write all the things I've done on my new phone.