I hope there's another Mr. O who can explain that...
I don't think +R vs -R is the problem here, as everything that has been done with Fab will play except for these two films.
I read not that long ago about a new scheme called the Versance Watermark, currently in use on only certain audio DVDs. Without going into a lot of details, what happens is, that if certain, select bit groupings do not come to a pre-determined total embedded on the DVD, the play function of the DVD is disabled after about 30 secs.
But then, the films do play on their computers, so that may not be the problem, either.
I would definitely slow down the burn speed, and see if that makes a difference.
Please let us know.