Check to be sure the magazine spring is right way around, and not bent, twisted, or kinked.
Check the follower to be sure it's not bent out of shape.
Make sure the mag isn't TOO lubricated. There should be just as thin a coat of lube on the inside of the body as possible.
Check the magazine for dents or a bent/crushed magazine body.
Check for broken welds that are allowing the body to separate or spread.
Check the overall length of the ammo. If the ammo is slightly too long, it will cause sticking.
If all else fails, buy a new magazine. Sometimes this is the only "fix".
Remember, this is a used ex-military firearm, and it's possible the magazine was damaged.
There is NO difference between the Walther POST-war P-38 and P1 pistols or magazines.
Same gun and mag, just one made for German police and commercial sales, and one with military P1 markings for the West Germany military.