Just a moment...
After shooting some clips and experimenting with the various recording modes, new shooters frequently toggle over to playback mode and find that many of their clips aren't shown. Where did they go? Are they lost? Did they fail to record? Is there something wrong with your SD cards?
No, none of the above, and there's no reason to panic. What's happening is a by product of the camera having multiple different recording modes available. In playback mode, the camera doesn't automatically switch modes, so it will only play back clips that were shot in the same mode as it's currently set in. What this means is, if you shoot some clips in MOV format, and some more clips in MP4 mode, and another couple of clips in AVCHD, and then you go to the thumbnail screen to play them back, the only clips you'll be able to play back will be the AVCHD clips you just shot! The MOV and MP4 clips won't even show up. Now, there's nothing wrong with the other clips, but the camera was last set in AVCHD mode, so that's the only mode of clip that's available for playback.
Resolving this is really very simple; just change the playback format to match the clips you want to play. Press the movie camera icon in the upper left of the thumbnail screen and choose MOV or MP4; then you also have to choose what frame rate and resolution you shot in. If you shot some clips in UHD and some others in FHD, you can't view them all together, you have to pick which ones you want to see and change the playback mode for each recording mode. (And no, before you get your hopes dashed, the "VIEW ALL" doesn't mean it will view all the modes, it means that you're viewing all dates that clips were shot on. You still have to individually choose the different recording modes.)
Furthermore, if you shot some clips in 60Hz, and other clips in 50Hz, you'll have to go back to camera mode to change the SYSTEM FREQ to the compatible setting for the clips you want to view. Sounds like a hassle, but in reality it's not a big deal. Usually you only change recording modes when you're experimenting, but once you settle down to shoot a particular project, you'll usually pick one recording mode for the entire project, so the mode-changing issue becomes largely irrelevant in actual practice. Just remember that if you can't see a clip's thumbnail, that just means you need to change the playback format before being able to play the clip back.