Appium 1.8.1 createSession command to WDA is always timeout at first time

i am doing ui auto testing using Appium (iOS platform), what i did :

1. one app without system notification window pop-up , everything is ok.

2. the same app with system notification window pop-up, i found error in log : createSession command to WDA is always timeout at first time but i works at second time .

i tried it again and agin , it always blocked at code "webdriver.Remote(server_url, desired_caps)", the corresponding error is :WD Proxy Error: timeout of 240000ms exceeded.

but it works well after trying to connect again, hmm, can somebody help to explain it ? thanks a lot.