we are currenty working on a big project and because of the performance we need to set the stage.quality to medium in one part of the application...

the problem was the following:

show app part 1: stage.quality = high with a lot of sprites and textfields...

remove app part 1

show app part 2: stage.quality = medium

change back to app part 1 without changing the stage.quality back to high

now, some of your textfields won't show up..
if you set back the stage.quality to high when changing, the problem is solved...

sorry for the complicated description, but it is a strange problem, only occuring on some textfields... hope i can help somebody, having the same problem...