you have to save the slo-mo proportion of the clip first. think of the slo-mo as an overlay. your phone records at a higher FPS but you can adjust where the slow motion takes place with the slider. Well, after making that adjustment the video needs to be finalized. You can do this by click the (...) icon in the upper right corner and processing the video with iMovie or iMessage or email it to yourself. Unfortunately Apple does not give a final save feature so you have to use one of the methods mention above.