If there's any way you can upload your project and the media it relies on to a file sharing service, it may help someone to reproduce it. A bug can't be squashed until it can be reproduced. Posting a mediainfo report (https://mediaarea.net/en/MediaInfo) of your phone media might help as well.

Sorry it has taken a while to get back to you. I have been troubleshooting this issue myself...

First, I appreciate your willingness to help. Unfortunately, I can't upload the video material because I don't own it (belongs to a friend).

However, through a large amount of testing, I have significantly narrowed down the issue. Originally there were a mixture of h.264 MOV files and h.264 MP4 files in the mix. The MP4 files were decoded by the Vegas "compoundplug.dll" plugin module. These WORKED FINE and had no visible limit on how many files could be decoded and played on the timeline, so this is a non-issue.

The problem seems to be with the h.264 MOV files, which mostly seemed to come from a variety of Apple devices such as iPhone 4s, iPhone 6, iPad, according to the metadata revealed in "mediainfo". Some of the files did not have any discernable metadata. Anyway, *all* of these files are being decoded by the Vegas "qt7plug.dll" decoder. This is what is choking. The behavior is as follows: If I drag in 14 of these video clips, comprising about 6 minutes, into the Vegas Pro 14 timeline, they seem to work as expected. However if I 3 more, for a total of 17, the "problem" occurs. The problem being the fact that scrubbing the timeline only reveals "black" in the preview window, not the intended video from each clip that was plainly there and visible before adding the extra clips to the timeline. So I could add up to 16 clips, and at # 17, the timeline goes bad and starts playing "black". After this happens, a person has to drop below the threshold of 16 down to 14 clips before video will start playing again. This whole sequence of clips amounts to no more than about 10 minutes (actually less) max, BTW.

So there appears to be a problem with the qt7plug.dll decoder being able to play more than a certain number of h.264 MOV files. This issue is not confined to one system, either. I have several Vegas licenses and have verified this behavior on at least TWO of them. I notice that this file is included with Vegas Pro, DVD Architect, and Sound Forge (11). On my system, there are 3 different versions associated with each of those 3 applications as well. Maybe one of them doesn't have this bug but I haven't done any such testing yet. I have turned on and off the GPU support (and restarted) on the "video" tab of Vegas Preferences. I have also tried upping the dynamic preview RAM. Neither of these settings have affected this problem. I have also tried adjusting some of the Project Preferences, such as deinterlacing, resizing the video, different output resolutions etc. I now believe this is an issue with the qt7plug.dll file rather than any Vegas "settings".

Has anyone else run into this issue, and is there a solution? The only workaround I have right now is to take small numbers of the source files and transcode them into a series of intermediate files in order to get them out of that format, before I can edit them. Due to the content of the sequence this will be a inconvenience to be sure but I can't think of any other options at this time, due to the different sizes and orientations of the many source clips.

Looking forward to hearing back from the community on this! Thanks, TMTV

@TMTV did you try to enable the quick time plugin? By default in Vegas 17 it is disabled, the use of QuickTime is depricated. You can re-enable it in the: Preferences ==> Deprecated Features tab ==> enable quicktime plugin.

Update: this will not help you, I just noticed in your post you are using Vegas 14...

Thanks for the thought, Bitman! There appears to be a problem with the qt7plug.dll decoder not being able to play more than a certain number of h.264 MOV files sequentially on the timeline. It acts like a memory management issue within the plugin but that's just a guess.

@TMTV have you already taken the route of using something like handbrake to convert the mov files to mp4?

@TMTV have you already taken the route of using something like handbrake to convert the mov files to mp4?

Thanks for the suggestion, Mr. Harrison! What I finally did was to use the batch transcode feature of Vegasaur and convert all of the problem AVC codec - MOV files to Magix Prores MOV files. This was of course a work-around and unfortunately didn't actually "fix" the problem, which seems to be a bug in that DLL file or related code. I wish Magix would comment on this issue. I can't believe that I'm the only person whom it has affected since these are Apple iPhone (mostly from a 4s) files. It would be interesting to learn whether this issue has been fixed in later versions of Vegas (mine is 14). I can post the "MediaInfo" report if anyone is interested, otherwise I won't take up the space. ;-)