AHA!
i think i have figured out why mine are disappearing.
i am clicking on a wall multiple times. in doing so i am associating the dimension string to that wall two times. in most cases i am doing it to dimension the stud thickness at the end of a wall. whenever i correct another dimension it seems the program "auto-corrects" all of my dimension strings to NOT associate with a wall twice. i think archicad 10 is extremely touchy with how you associate the dimension strings and will "auto-correct" anything it deems wrong...any similar circumstances?