So if the problem is minimized by switching the heads, but still present in the same staple position, I'd say you should look at the clincher next.
I had a DBM 120, which I think is similar to your 150. The clincher could be removed by loosening two nuts and lifting it out. Open it up and check for free movement of the swinging staple-curving anvils. BE CAREFUL! Tiny parts lie within...