Teachers may have a hard time allotting time beforehand. It is very time demanding to give diagnostic tests, and thus may only be achievable a couple times per semester at most. Sometimes the assessment may have been developed poorly and thus may give false results, thus guiding the lesson planning in the wrong direction. Finally, since this is not something students study for, they may be in a poor mindset at the time of testing, and thus perform poorly, further invalidating the results.
