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I unequivocally recommend the foolproof method BUT if you still ... have trouble improving despite constant fool proofing then there could be ...
Fool-proof first and keep practicing formal logic diagrams since those are the most trainable. That gap does strike me as a bit odd but it'll close over time.
How long have you been studying? It's normal to study a year for this test!! Foolproof the LGs. Have you gone through the course? Do you truly understand the concept?