Either the globe is not seating firmly in the socket, or the socket has minor corrosion in it which prevents the contact until you tap it. Remove the globe, check it for signs of corrosion on the sides and on the base contact. Check the condition of the socket, clean out any corrosion you find, spray the socket with WD 40 to assist in the cleaning and removal of crap, then re-insert globe. This should do the trick.
Regards
pj
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