They will come out when they are hungry. Presumably they are new and young? Give them time. Your clowns will develop and become more bold.
What you can do if you are worried, is to get some pipe (not see-through is best) and put it into the water straight down to the bottom with the end by the clown's hiding places (not smacking their heads or banging their cave, just in the rough area). Chuck the worms in the top of the tube and they will float down the tube to the substrate without the tigers being able to get to it (they will see you trying if you use a clear tube though). Clown's don't mind picking food out of plants or the gravel either, whereas tigers are more open water feeders, so don't be afraid to send some worms into hard to get to places to encourage them to come out.
Feed some sinking flakes too, and break up some prawn too, they love it.
If you've got lots of lighting, try reducing it for a week or two (unplug one or two of the tubes), to help them feel less threatened.
With your setup I reckon that you could add a nice plec without any problems, if you wanted more variety.