Flight data do not conform to mission plan

Your images leave out some of the important parts, like the signal strength indicator. If you are using Active Connect you need to maintain a super solid connection for the triggering to take place reliably. If the signal even gets down to 75% it might not work well on the P4P. 

Another thing that happens during the summer is that the iOS device gets hot and starts slowing thing down, especially in direct sun. Keep it in the shade and keep it cool. 

If none of this seems to work you should use the Connectionless Mode which makes it so the aircarft/camera is triggering the camera, not the app. This makes it so your images are not all collected while traveling the same speed but at least you get your consistent overlap.