Well, simply put. Does the sprue project below the base of the bullet at all? If so, this could be your problem. The bullets base needs to be perfectly flat for the gaschecks to seat perfectly straight. If you have projecting sprues, try breaking the sprues with a gloved hand. Lots of guys still beat their moulds with sticks to break the sprue, and if you catch it at an angle you can spring the sprue plate upwards a bit. This often causes a bump on the base that makes it impossible to seat a gascheck squarely.