My wife shoots a SKB 150 20 ga and we usually use light reloads. Today we had several problems where the 2nd barrel wouldn't fire. I am pretty sure it is due to the light reloads not tripping the inertia system. The gun doesn't have a recoil pad so that isn't the problem.
Has anyone altered or adjusted their similar gun to accommodate light loads. At the end of the round she tried my 20 ga W 21 and hit 2 of the tougher targets on the course and said wow it swings itself.
SKB's are popular with the cowboy shooters and many have had a gunsmith convert the gun to mechanical reset rather then inertia.
Having said that, if it only happens sometimes, look for other possibilities, including maybe upping the load just a little more, gunk inside or maybe the gun isn't being held tight enough to the shoulder on every shot.