You probably have the head lined up slightly off the bolt hole. Try dipping the head bolt in some motor oil, and wiggle the head a little to try and get it to catch.
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
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