Some clowns will never host anemones. It's hit or miss. I hope that your water parameters are stable enough and you have enough lighting for an anemone. If it gets sick or dies in your tank, there is a big chance that it will kill everything else in the tank too. They are very risky animals to keep, especially for young tanks and small tanks (both of which yours is). Good luck.
One trick you can try is to cut out some pictures of the types of clowns you have from a magazine. Then tape the pictures to the glass of your tank where the anemone is. So that when the real clowns look at the anemone, they see the pictures of the fake clowns taped near it. That may convince them to go check out the anemone. It sounds weird, but supposedly works often enough for me to hear it over and over again.