Catching up is hard to do

Students who have realized that the end of the trimester is only a few days away are now bringing in missed assignments they think will bring their grade up.  However, our standards-based grading only counts tests and quizzes for grades, so showing us their homework doesn't affect their grades.  Other students are asking to retake tests, so we have set up times before and after school, as well as during lunch tutorial, for students to come and take a different version of their unit test.  It seems odd to me that students who go to the trouble of taking another test have not made an effort to study the questions they missed on the first test. In some cases, the students miss questions that are very similar to ones they got right the first time.  Or they'll get the same questions wrong that they missed the first time (not every question is different).  I wonder if it's due to parental motivation that the students are retaking tests; it hasn't brought up anyone's grade so far.  We had one student planning to retake the test tomorrow, but now school is cancelled due to snow.  There's always next trimester!