Failed the lab science certification twice — what am I missing?

I'm honestly at a loss. I've taken the MLS(ASCP) twice now and keep scoring in the low 80s when I need a 90+ to pass my program's requirement. I've been using a commercial study guide for about three months and clearly something isn't clicking, especially in chemistry and immunology. My microbio sections are fine — it's the chemistry calculations and the serology interpretation questions that keep tripping me up.

What finally worked for people who struggled with those sections? I've started supplementing with practice questions online and found the Laboratory Science Clinical Chemistry 2 practice test helpful for identifying gaps, but I'm not sure if I'm studying the right way after. Do I just keep drilling questions or should I go back to the textbook when I miss something?

Third attempt is in six weeks. Any study strategies, resource recommendations, or honest exam tips would mean a lot right now. I'm a non-trad student with a full-time job so I have maybe 10-12 hours a week to dedicate to this.