Hi,
I've put about a 1,000 Rounds of Sellier and Bellot 124gr 9mm through my 1937 S/42. Together with a MecGar magazine this pistols functions like new (well, even better

).
I agree that the advice above is sound:
-Clean the gun thoroughly.
-If that doesn't help, try a fresh MecGar magazine.
-If that doesn't help, try a Wolff spring replacement set.