adsp21569 for channel configuration base on the 21569sdk4.1

hello! there is a question when i use adsp21569 for Dolby atmos.i transmit my instruction that configure the output channels as 5.1.2 ,But there are only three audio outputs out of four output channels(I2s).I configured this channel according to the SDK(21569SDK4.1)and the test audio file is True HD.who can tell me what the problem is ? thank you very much!