I'd been using Motion 4 for about 6 months, and encountered this problem on the last project I did: When I pressed COMMAND-R to do a RAM preview, it rendered all but the last few seconds of a 4-minute piece. However, when I exported it as a QT movie, it rendered all the way through.
Just this week, I started working in Motion 5 and have a new project that is currently 14,400 frames long. But when I did a RAM preview, it only rendered the first 699 frames. As a test, I tried exporting it, and it did render all the way through.
Any light anyone might be able to shed on this would be greatly appreciated.
The length of a ram preview is entirely dependent on the amount of ram available in you system. Once it fills up that ram it no longer has anyplace to render the frames to. My guess is it has filled up your ram and that's why it's not ram previewing the entire project. I would try setting a work area in and out on the timeline and then previewing in section defined by that work area.