The issue is too much repetition after a topic's mastered, constituting a waste of time.
IME you need to get 3 answers right to master a topic, and then it gets reassessed once or twice in the review the next time you log in (my son hated that one, and ColinsMum showed me how to bypass it), and then once in the next assessment.
It might also show again once in a later assessment -- my son managed to screw up on one thing that killed a 3-4 topic dependencies strand -- but not that many times, since the assessments have about 30 questions, 20 of which testing the last 20 topics mastered and about 10 of which fishing back (review amongst previously mastered topics) and forth (checking for mastery on never introduced topics).
As the pie grows the likelihood of having a topic show up more than 4 times (3 for mastery, 1 for assessment) decreases.
Like ColinsMum there was a time when repetition was an issue for my son, as he slogged through stuff he didn't like and his tendency to make one careless error that triggered a spiral of frustration + more careless errors + more frustration led to... well, a lot of frustration. Skipping the next session review helped. Not letting him do more than 2-3 topics in a session helped more (and bigger topics help with that too).
What level did you start at? Have you considered moving him up the curriculum? All the elementary math levels include pretty much everything in the levels below, so he might get a better fit with the initial assessment (which, as mentioned above, cannot really test *everything* in a reasonable time frame).
