New calendar entries are automatically modified

I have just started having this issue. I create a new calendar entry and type in the details and close to save. I come back later, within the same day, and there is a blank entry - the data has been removed.

If I double click I see "New Event" and I need to select this and change the details. So far, if I make the change, it does not modify later.

Has anyone got ANY ideas ?

OSX 10.9.5

Kelvin