In the past, the US Navy frequently made port calls to Qingdao, Hong Kong and Shanghai. However, the current trade war and pro-democracy protests in Hong Kong (which China accuses the US of inciting) have increased tensions enough for the Chinese government to deny an American warship permission to visit a port in Qingdao. Besides Qingdao, permission was also denied to for a port call to Hong Kong. China most likely denied the port call to Hong Kong as it may have been supportive of the protesters.
Other reasons for the current denials could be a response to the trade war and the recent $8 billion dollars sale of F-16 fighter jets to Taiwan.