Maybe, but I suspect you are just expecting more than it will deliver in some circumstances. It's an engine brake (Jacobs type) rather than exhaust brake, and the two positions brake with either 3 cylinders or all 6. Neither position has a lot of effect at high speeds because the tranny can't downshift much without over-revving the engine. As you slow down (use the service brake to help if needed), it continues to downshift and you get more and braking effect.
You should see the indicator on the shifter go to "2" as soon as the engine brake engages, but the actual gear in use will depend on the mph speed. Above 60 mph the tranny stays in 6th gear and you don't get a lot of effect. The tach will show an RPM increase, though, and as the speed falls the tranny downshifts to 5, 4, etc. and you feel greater braking.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL