I applied for a 90 days business visa...but was only given 30 days visa

had invitation letter for 90 day visa single entry and applied for same..............but i was only given a 30 day visa....do i have to leave china and go to hong kong and apply for another 30 day visa.................or can i have it extended whilst on the mainland (foshan)......................i need more than the initial 30...........why was i only given 30 day.........how do i get around this limitation