First, check whether it's really flat
One test score isn't a trend. Look at your last two or three full tests by section, not just the single overall number, since a few points of swing test to test is normal noise. If your section scores have genuinely stayed within the same narrow range across several tests spaced a few weeks apart, that's a real plateau worth diagnosing rather than a random dip.
The most common cause: not reviewing deeply enough
Real review means redoing the reasoning for every miss and writing down why it happened, not just glancing at which letter was correct. A student who checks the answer key and moves on sees the same skill gap show up again on the next test, because nothing about how they approach that skill actually changed.
Second cause: drilling broadly instead of targeting
Rank your misses by how often each skill appears on the test multiplied by how often you get it wrong. Spreading study time evenly across every skill category, including ones you rarely miss, under-drills the small set of skills that are actually costing you the most points. Fixing your two or three highest-ranked skills usually does more than touching everything a little.
Third cause: treating every miss the same way
Redo a missed question untimed later. If you get it right, it was careless, and the fix is a slower, more deliberate check on that question type, not more content review. If you get it wrong again untimed, it's a real gap, and the fix is re-learning the rule before doing more repetitions. Getting this backward is common: drilling more of a skill you already understand doesn't fix a rushing habit, and re-reading a rule you already know wastes time you needed for real practice on skills you don't.
Fourth cause: testing too often without recovery
Repeating full practice tests without gaps to drill the misses in between means the same errors show up test after test, because nothing changed for that skill in the meantime. A cadence of roughly one full test every two to four weeks, with real drilling in between, gives each skill a chance to actually improve before it's tested again.
A reset process if you're stuck
Take one full test and this time review every single question, right and wrong, for why you got it right or wrong. Sort every miss by skill and by careless-versus-gap. Pick your top three skills by frequency times miss rate. Drill each with 8 to 10 questions today, then again in about three days, then again in roughly two weeks. Take your next full test after that cycle, not before, so it actually reflects the work you did.