Congrats on your first year. The clients don't pre-book because they know they can get in whenever they want now. As you get busier, that will change. With Spring coming, now is a great time to say that with the warmer weather, they should book ahead or you could be full when they want the groom. Another thing you can do is make a call at about 4 weeks saying that you're filling up, and would they like to book Fluffy's appt now. At my home shop, I have about 95% pre-book. Makes it nice because you can plan your weeks and you aren't interrupted by a lot of phone calls.
Old groomers never die, they just go at a slower clip.