I'm using kdenlive 21.04.3 on Fedora 35. I'm building a slideshow with a mix of pictures and video clips, and while I can drag the former to the timeline, the latter fail, at least for all MP4 files. The files in question all load properly and are playable in the clip monitor, but dragging them to a video track triggers no response--it just hovers above the timeline without reaction, and releasing the mouse button just dispels the dragged object; there's no error message or any cue that something is amiss. In contrast, I've successfully (and surprisingly) loaded an MKV file from the project bin onto a video track without trouble. I've looked at the kdenlive manual and video tutorials and it looks like a brain-dead task, so I don't think I've overlooked anything. And of course Google has no suggestions to make. Please advise.

Not sure, if I really can help you, but:
- try the newest appimage (22.04), there are many bugs fixed,
- in 21.12.3 I have had such a similar crazy effect in dragging something to the timeline.
Maybe, you should pick the clip in the clip-monitor at the blue field containing the clipname at the top, then it worked for me.
If that will not work, you should check, if your project-settings (video params) are compatible to your failing clips.

See if your video has audio...
If your video has audio then you need a video track and an audio track because probably both are joined together.