[Stratos]

I haven't have this happening to me personally so I don't know exactly what it can be, but are you sure you don't have him in Follow Mode or anything similar? Or anything that makes him do something that he can't do instead of bashing chests. Check his Character sheet and see if he's still wearing his gloves. Maybe they are somehow un-equipped.

At level 10 Monks get the Ki Strike which means that their hands are considered +1 weapons when it comes to eating through damage resistance. If I were to make a guess about the problem, I would say that you're trying to hit a chest which have so high damage resistance that you need a +1 weapon to punch trough. Where as you are able to hit the chest the henchman refuses if he/she doesn't have a +1 weapon equipped. Now that I think about it, I remember I have had this problem myself. When you hit the chest; does a message, saying that you can't damage it with your current weapon, pop up in yellow in the text bar at the bottom of the screen? If so it might be the case that Grimgnaw doesn't have a magical weapon yet so he wont attack the chest. Try with another henchman who does have a +1 weapon and see if that's help.

Finally, do you have the sáme problem with all chests or just a few?