I am using kdenlive on several openSUSE systems (Tumbleweed version 21.08.0, Leap 15.3, 15.2).
I have a number of mp4 files as clips in the project bin. When I drag one of these clips in the timeline, I do that to V1, nothing happens. I can only drag the clip to A1 and it shows the sound miniature. The Clip Monitor shows the video frame, but the Project Monitor remains black.
I have the same problem on Leap 15.3 and 15.2, most likely with other versions of kdenlive and related libraries/programs.

Question: what is wrong on my system and how do I solve it?

When the clip is selected in the project bin does the clip monitor play the video?
When you drag the clip from the bin make sure you are not dragging the tiny speaker icon.

bigboss97 wrote:When the clip is selected in the project bin does the clip monitor play the video?
When you drag the clip from the bin make sure you are not dragging the tiny speaker icon.

I tried this morning on openSUSE Tumbleweed and now this problem is gone. Most likely because I updated the system yesterday late and started it only this morning.
So I decided to try Leap 15.3 and 15.2, also after an update. Also on these systems, the problem is gone.
There was no update for kdenlive, so most likely a utility or library got renewed.
Unfortunately I could not really answer your question.

fdekruijf wrote:I tried this morning on openSUSE Tumbleweed and now this problem is gone.

Just for information, whenever you want to drag audio only you can drag the tiny speaker icon of the clip in the bin, the film icon for video only.
They are quite useful. I use the video only icon a lot.

fdekruijf wrote:I have a number of mp4 files as clips in the project bin. When I drag one of these clips in the timeline, I do that to V1, nothing happens. I can only drag the clip to A1 and it shows the sound miniature. The Clip Monitor shows the video frame, but the Project Monitor remains black.

Oops, now I'm having a similar problem, too. I've extracted a video from YouTube which is played on VLC without problems. When I drop it into Project Bin it becomes an audio track only and it doesn't even appear in Clip Monitor.
I'm using v21.04.2 on Windows 10.

The problem appears to be present when only the openSUSE multimedia packages are installed. Replacing these packages with the packages from Packman solves the problem.