First, I would like thank Gantt, who made "Download YouTube Videos as MP4" (DYVAM) available.
Before telling the problem, let me provide some background information.
In 2013, for the resolutions 480 and 1080, YouTube adopted a new format called "MPEG-DASH". This new format was incompatible with many downloaders, and they became no longer able to download 480 nor 1080. Gantt worked hard to make DYVAM support the MPEG-DASH. He succeeded in making DYVAM download 480 and 1080. However, due to the technical difficulty, for DYVAM to download MPEG-DASH, the video and audio had to be downloaded separately. After downloaded, the separate video and audio can be merged by an MPEG-editing program called "ffmpeg". However, only advanced users can comfortably use ffmpeg. Ordinary users cannot use ffmpeg. Thus, Gantt decided to hide the feature of downloading MPEG-DASH (480 and 1080). By default, the "Download" button of DYVAM hides 480 and 1080.
On Firefox, from about:config, by setting "download-youtube-dash-enabled" to 1, the resolutions 480 and 1080 appear for the "Download" button of DYVAM. For example, they appear in the following manner:
MP4 480p - no audio (80 MB)
MP4 1080p - no audio (300 MB)
M4A 256kbps - audio (35 MB)
Note that, if the video does not have 480 or 1080, that is, if the gear/dial icon (the Settings option) of YouTube does not show 480 or 1080, then DYVAM cannot display nor download them, either.
Now, here is the new problem, which I only recently have encountered. There is a series of video consisting of episodes #1 through #42. All the episodes have 480 according to the gear icon. With "download-youtube-dash-enabled" set to 1, DYVAM successfully displays 480 under the "Download" button for most of the episodes. However, for only eight episodes, DYVAM fails to display 480. Let me repeat that the gear icon shows 480 for all the episodes, including the eight episodes for which DYVAM fails to display 480. Note that reloading of the pages does not solve this problem.
I would like to provide the URLs of the videos I am talking about. However, I am not sure whether I am allowed to write URLs in posts in this forum. So, I omit the domain of youtube.
The location of the series is
Among Episodes #1 through #42, the only ones for which DYVAM fails to display 480 are #11, #12, #17, #19, #24, #25, #33 and #42. Note that the list item numbers do not necessarily coincide with the episode numbers.
Let me provide some locations of the episodes, for example.
For the following, DYVAM successfully displays 480:
For the following, DYVAM fails to display 480:
Gantt, please investigate this problem.