How to search the subelement in ios?

i’m useing the appium like this
el = self.server.find_element_by_name(“aaa”)
el.find_element_by_name(“bbb”)

however, the element el can’t find the subelement with the attribute name “bbb” , how can i fix this, please?

i have tried iphonex(ios version 12.0) and iphone6 plus(ios version 10.3.1), both failed
appium version:1.7.0