By David Shepardson WASHINGTON (Reuters) -The U.S. Federal Aviation Administration (FAA) has recommended airlines operating Boeing 737-900ER jets inspect door plugs to ensure they are properly secured after some operators reported unspecified issues with bolts upon inspections. The recommendation issued late on Sunday follows the FAA’s grounding of 171 Boeing 737 MAX 9 planes after the Jan. 5 mid-air cabin blowout of a door plug on an eight-week-old Alaska Airlines MAX 9 jet. Boeing’s shares fell 2.5% in premarket trading on Monday. They have fallen 17.5% since the beginning of the year. The 7…