Does your game include a lot of group interactions or individual? Since you have quite a few players, if they are waiting to take their turn, they may not seem as engaged as they would be while working together on a task. Maybe try shifting towards group combat or group quests to keep everyone focused.

You mentioned that 2 players instinctively go to violence. If that is the way their characters are built, you should let them indulge in their instincts. As a DM you want the plot to go a certain way, but you can't dictate exactly what everyone is going to do at every turn. They should be allowed to make decisions based on what they are interested in doing. Now... if they do something that is against your plan and it could potentially ruin a future plot, there could be repercussions, but the player should still be able to take that path if they so choose.

You also mentioned you are all close friends. Can you discuss with them individually their frustrations to see if they can give you any clues on what it is that is bringing them down? Maybe they don't want to provide harsh feedback in front of the rest of the group.