Märklin turnout 74490 doesn't activate every time when switching (green to red)

The arm from the turnout might be catching or binding on the metal housing (the side which has the white pins sticking out).Remove the electric turnout and hold it with the white pins facing down, slide the arm in and out.  If you feel any resistance or a slight bump then you might want to slightly pry the metal housing outward on the side with the white pins.  Slide the arm in and out again to see if there is any change, if there is no more resistance then the problem should be solved and the turnout can be install once again.

Red arrow shows where the metal housing is scraping against the plastic arm.

Red arrow shows the end result of prying the metal housing outward slightly.
There is a gap between the metal housing and the plastic arm.

Red arrow shows the part of the plastic arm which scrapes against the metal housing.
Metal housing is indicated by green arrow.