I'm a SIG armorer so, hopefully, some of this will help. First, make sure that you are not resting your thumb on the slide stop. Often, especially with a new weapon, malfunctions are induced by the shooter. Second, test your weapon with each mag and see if the problem occurs with only certain mag's. If so, you may consider replacing that mag's spring or follower or both. If the problem continues, you may need to replace the slide stop tension spring. This is mounted on the left side of the locking block and you'll have to remove the take-down lever and locking block to get to the spring (I really doubt this is your problem).