I've been having trouble getting a green phantom pleco to eat. As others have suggested feed right before you go to bed and take the food out when you get up. Mine took to zucchini first, and a lot of others suggest to start with zucchini. If you know it's eaten wafers in the past you can break a piece of wafer off and stick it in the zucchini. Seachem makes a garlic liquid that's supposed to increase appetite, so you may want to try that if it gets really stubborn eating.
Keep a close watch on the eyes and belly. The eyes should be buldging from the head and the stomach should not look sunken.
Also watch the poop. White or clear poop indicates trouble in the digestive system. Otherwise it should be close in color to what it's eating... Ie if it eats zucchini it should be light green, if its been eating wood it will be brown. Driftwood is good to have as it can help "reseed" the digestive tract and get things going again