Check your "F" series magazines for clearance between the baseplate and gun. Remove the baseplate and sand down the top of it until there is a slight wiggle when the mag is installed; this facilitates fully locking in and correct feeding.
Just had a IDPA shooter send in a Sigma to S&W because his "F" mag would not feed correctly. They replaced the mag, but they also took out the polished sear block I did for him and replaced it with stock, thereby losing his trigger job. He is now pissed that I will not polish his new sear for free.