Working in a Motion project and I've places the "Line Glow" particle emitter in the project. The project is fairly simple so the only thing I did to it was move the position of the line. I save the project, play the project over and over, adjust other things, even export the video and everything is fine. If I leave work for the day and shut off my computer, the next time I open that project the "line glow" particle emitter shows up in the timeline, everything is there but it will not show up on preview. Even if I export the video it will no be there. The "line glow" is the only thing that disappears visually in the video but remains in the timeline as though nothing has changed.
I had the exact same problem. I think I cracked the case:
The 'glowing line' uses a blurred circle as a particle source. This is the SHAPE, not the PARTICLE EMITTER. I had duplicated the glowing line and used it in 2 places in my animation. Both of my emitters referenced the same shape as the particle source. That is the problem. Ensure that each emitter has it's own 'blurred circle' shape, and that each 'blurred circle' shape is in the same group as the emitter.
Duplicate the 'blurred circle' and move it to the same group as the particle emitter. Then, drag the 'blurred circle' shape to the 'particle source' well on the emitter tab. This should fix the glowing line immediately. No need to save, close, re-open, etc.
If you only have one emitter, duplicate the 'blurred circle' shape (again, this is the shape, not the particle), drag it to the 'particle source' well and delete the original 'blurred circle' shape.
This turned a very unreliable graphic into a rock-solid one. Hope it helps some of you that might have the same problem.