Have you taken any other NBMEs? Is your UW% 1st pass, timed, and random?
Without the above knowledge, I can Only give you some general thoughts/advice. I would go through 18 and figure out why you missed what you missed. Knowledge deficits, changing answers/second guessing yourself, missing info or misinterpretation, etc. The NBMEs feel really different than UW to me, with the questions being much more vague and I really have to focus on not panicking/psyching myself out and just going with my gut to do well on them. UW on the other hand gives beautifully crafted stems to where if I miss something it’s almost always due to knowledge deficit. NBMEs I noticed I miss stuff more due to being tricked, or overthinking something and tricking myself, which hardly ever happens on UW for me. And there isn’t as high stakes a feel to doing a block of UW versus a test you are hoping will predict a good score for you.
Good luck to you.