Unable to get slide to advance when media completes

I just had the very similar problem.  I have an 8 second video (from a file) on my timeline. No matter what I do, Storyline stops the timeline at 7.5 seconds. So while Storyline can't tell where the video ends, it can certainly tell when the video is 0.5 seconds before the end. I'm sure there is a reason for the "when media completes" condition, but I can't figure out why.  Here's how I solved it:

I created a cue point (1) on the slide just before the point where the timeline stopped advancing (mine is 1 second before the end of the video on the timeline. I then added another cue point (2) after the video end on the timeline. I then added the trigger:
Jump to cue point Cue point #2 (X.XXs) on this slide
    When the timeline reaches cue point Cue point #1 (X.XXs) Xs

Then use a trigger to jump to the next slide at Cue Point #2. If required, you can paste an image of the last frame of the video just before and lasting until after Cue Point #2 to make it look like no code is being executed. Or you can fade out the image and jump to the next slide when the timeline ends.

I hope this helps!