Jiffy,
You don't say how long you've owned it...if you just purchased it, there's always the risk of lost business with an ownership change. On the other hand, if you've owned it all along, you've got to figure out why you're losing business ASAP. What have you changed? Food quality, menu items, help, prices...any or all of these things could play into it. Has a competitor opened recently near you. Or, more likely, are just subject to a weak economy. As people get squeezed financially, they will rein in discretionary spending and eating out is usually a choice.

If you know the customers that have stopped coming try to ask them why they stopped coming? I'm only assuming here, but I'll bet many will tell you they can't afford to eat out like they once did.
Good luck
Denny