Calendar Reminders Dissapearing

Over the last few weeks I have random calendar reminders just disappear. I actually saw one do it today. I had three reminders and one of them just disappeared before my eyes. I have one calendar I sync with and am not sure if I am adding the items on my calendar through my PC or my phone but I never had this problem in the past. I manage important things this way so this is concerning to say the least. Anyone know what the issue is?