Make sure it is well lubricated and action is clean - no globs of WWII dirt on slide/ toggle/ spring/ etc. Every one of the Luger's I bought had parts that had NEVER been cleaned and lubricated. Some required attention to work properly. Then shoot it with a firm wrist. Also insure that the main spring is not a wrong size replacement.
http://forum.lugerforum.com/showthread.php?t=34792