Most of the time they extend when they are in feeding mode. I have one in my main tank that doesn't extend very much , but I have 2X250watt halides. The other one in my pico is under PCs and it extends several inches. I think the amount of lighting has a lot to do with it. Try target feeding some brine or mysis it may get it in feeding mode. Also be careful of those tentacles. When they extend they can sting other corlas.