iPhone camera sometimes doesn't record

Sometimes when I hit Record on the default Camera app it changes to the recording interface (so no more mode menu at bottom, etc) but when I hit the red button again to end the recording I find that nothing was actually saved. On further testing it seems when this happens the number at the top doesn't increase and there's no flashing red dot next to it. There's ample space on the camera (several gb) for a recording to at least start, and it starts on subsequent attempts, but it's not going to fly if now and then something I thought I spent minutes of my time recording has gone nowhere. Advice?