Question about Outlook Express

I am loosing posts in my calendar. At first I questioned my own memory and wondered if I occasionally did something wrong. However, I have since then been extremely careful and still, from time to time, but way too often, post in my calendar just disappear. I d I do not seem to be able to trust it and I am about to give up on Outlook. I really need to find the reason and fix, because I have already missed important meetings and it just can't be the case that I am supposed to have to check more than one calendar to get to all important meetings.