Even if you don't add more clowns, it's helpful to have multiple caves/hidey holes, and plants around the perimeter that overhang the center somewhat. It also helps to have "dither fish." This means having small fearless fish that will cruise around the mid-top of the tank all the time. The clowns see the little fish aren't scared, and are more likely to come out.
I have one
Clown Loach(yes, I know, I know). I've had this one for about 12 years. I always feed him at the same time of day, or as soon as I come downstairs in the morning. I also have a few regular times when I usually come sit and watch the aquarium. For example, in the morning while having breakfast. If you make it a habit to sit about 4 feet away from the aquarium after you feed them, and calmly watch, or read, or eat, yourself, you will establish a routine. My clown knows who feeds him, and will come out and bustle around when it's just me(or wiggle his belly at me to beg; he's learned this works with me over the years). He hides from most visitors, but comes out when a friend who is his regular "fish-sitter" if I go out of town comes over. On weekend mornings, when I'm sitting watching the tank for a longer time, he expects that this means an extra treat of some sort.
However, if I move much of the aqua-scape around, all bets are off, and I'm snubbed for a week or so.
It may also help with new clowns like yours, to sit and watch in the evenings, with the aquarium light off, but the room lights on. This is if yours are particularly disposed to evening exploration.
Provide hiding places, dither fish, a routine, and be patient. :)