There are a number of reasons why you are unable to edit a particular diary event.
Usually it is because you do not have the correct security level to alter diary events. If this is the case please speak to your Manager or Account Administrator.
The second common reason is because the event has already been accepted by an engineer on his mobile phone. Once an event has been accepted by an engineer you are not able to delete or abort the diary event from the office. In this case you will need to telephone the engineer and tell him there has been a problem. The engineer can then abort the event from his phone.