Could be a number of things such as antenna orientation, particularly when flying overhead. If you are only losing video and not control signal, try going into the app and select a custom channel and wind your transmission rate down to 4mbps to make it easier on your device. With each app update, I found my iOS device was struggling with processing the data when flying so I cleared my video cache, selected a clean transmission channel and that fixed up most of my problems. I also junked the original controller antennas and fitted a boosted FPVLR setup and now the battery is the only limiting factor.