This is actually normal, to some degree. The trigger rides the cam down when you pull it, hence the pull will be stiffer. However, it should not be much. If it's very stiff, it means the cam is not unlocking enough, or that the trigger profile is wrong. Sanding the trigger to be a bit more curved, or using a lever with a bit less slop, is the solution. Tends to be a metal cam issue as well.
