iphone6 IOS8 static IP will change back to DCHP automatically

I got a Iphone6 with IOS8. I want to set it to static IP, I have changed it to Static IP, but after back to the wifi list and enter the information page of the wifi, I find it change back to DHCP by it self and all the configuration of the static IP are gone. I got other apple devices and those are works good on static IP, and I try to use different IP and some of them works on other devices but all can't work on this one. Anyone met the same problem and has any idea on it?