My grease pencil marks disappear several frames into the animation, how do I fix this?
It sounds like you are actually animating the grease pencil strokes (or your 'annotations').
If you draw on the same grease pencil layer on different frames it will add 'keyframes' and animate them. (ie - display your drawings on the frames you have told it to)
If you want the strokes to stay visible the whole time make sure you draw them on a different layer.
You can also activate onion skinning which will show you the previous and next drawings.
Hope that helps you out., raijkeraijke.