I've had mine two seasons now and never had a problem with it. Best unit I have ever used. How long have you had it? Are you sure your using it correctly? Which buttons are you having a "problem" with? The ones along the bottom, or on the side?
There have been a few machines where the some of the buttons have stopped responding, unfortunately. It's been on a MUCH smaller scale than the Northstar 6000/6000i's. Your first try to should be to call Navico in New Hampshire and request a fix, but if they tell you that you need to pay an exorbitant flat rate repair, PM me the SN and I will see if I can get you set up what's called a 'Minor Repair' where they bill only for the parts that need fixing. Good luck.
Call (603) 324-2042