Had the same problem & figured it out today. Its the rear baggage weight. Use the EFB to check the C of G on the page that shows gas , passenger/pilot weights . I had light Pilot/Copilot & heavy passengers. Took out the passengers, CoG still way back out of range until I reduced the rear baggage weight. After getting the CoG in range it was still on tail until I revved engine, which pulled nose down for normal take off run.