visa requirements such as invitation, flight ticket.

hi...My friends and I are going to zhuhai for printing exhibition this sept. i want to ask if we have to/must provide documents like invitation from the exhibitors/suppliers, flight ticket (to HK is ok?).
i have gone to China many times...before i didn't need to provide those documents for visa application. but last april 2012...i was denied by the embassy security...they demanded those documents in order to get the queue ticket....
please explain in details....thanx...