Jhieminga,
I've come across this problem on the B707 & the DC-10 (the latter being a lot less susceptible). It was usually caused by being parked on a slightly uneven surface, especially if the aircraft was stopped during a tight turn.
On the B707 the solution was usually to tow or taxi it to a more level area & on the DC-10 (where the problem was usually opening not closing) it was usually to tow the aircraft very slowly whilst trying the door.
Main-deck cargo door vent-door switches were often a problem on the '10 & the first place people would check if there was a problem with the door.
Cheers