why donot dm8168 ezsdk encoder generate I frame except first frame?

hi all,

         My board is dm8168 evm. I use ezsdk.Now i encode yuv file to h264 file.I set the I frame interval parameter  is 30.But the h264 file is only first frame is I frame.Other frame is not I frame.Why is it?Who can help me?Thanks.

Best wishes,

hjt