Hi, a 5-star rating (as opposed to 3-star) would likely improve the situation, so it is definitely worth trying
(for a quick test, you could use chips.jpg and stones.jpg targets that are available with our samples; I believe stones only has 4 star, while chips has 5 stars, but in any case they are both very good targets).
Another thing you may want to try is to change the camera mode to MODE_OPTIMIZE_QUALITY (as opposed to MODE_DEFAULT); in Unity this parameter can be set through the Inspector in QCARBehaviour (see Camera Device Mode Settings box)
Last thing: as you also say (device loosing focus), your issue seems pretty much related to the ability of the camera to keep focus on the targets;
you may want to try changing the FocusMode for your camera, and test the available options to see which ones yields the best results:
to change the FocusMode, you can use this API: