After adding clips together in Quicktime X (Mavericks), can't save and video goes black (does not display). What's going on?

I am attempting to add together multiple short video clips. All approcimately 1 minute long. All are .mp4, H.264, AAC 1080x1920. I open the first clip (as instructed in help), go to the "Edit" menu and select "Add clip to End" I then find the scond video and click "select movie." The clip is added but the two clips do not seem to splice together. Both clips go black and do not display any of the video. Audio is still present. I can still see that video is still somehow there as when I ask to display clips (instead of timeline) I see the icons in the timeline. The save menu is not an option as it is grayed out. I checked Activity monitor and Quicktime does not appear to be using much if any resources (as if it is processing the video clip and combining). Anybody else experience this? I've tried all the usual (updates to latest software, reboots, etc...). I'm kinda at a loss and really like for this to "just work" as Apple normally does (I've used Apple hardware and software for more than 28 years and I have to say this is one of the most frustrating problems I have ever dealt with and couldn't solve on my own). I appreciate any feedback and suggestions...