Try a different species of leaves, or even the same species form a different tree. You may also want to try putting a few different kinds of leaves in the enclosure at once to find out which species your stick insect likes best.  Sometimes phasmids may be happy eating a particular species for a long time, and then suddenly go off it. This may be due to seasonal changes in the chemicals within the leaves. It is important to always be sure that you phasmids are actually eating what you are feeding them. A good indication that your phasmid is eating is by checking that they are producing waste (poo or frass) which they drop on the bottom of the enclosure.