Previously added clips are visible in the preview, but after adding one it simply does not add up to the preview. Everything else is previewed fine, all the compositions, just this single clip behaves as if it was not sourced into the stream. Saving the project and restarting kdenlive, then reloading the project helps.
It seems that this report is related to very old unmaintained version. A lot changed since then, especially the timeline got a complete rewrite and it is likely that this has been fixed. Feel free to reopen this bug or create a new report if this is still happening with the latest version (https://kdenlive.org/en/download/)