Check for sap-sucking bugs. Not all of them hang out in easy-to-see places all day. They could be down by the soil or under perfectly healthy looking leaves.
Even if you don't see them it won't hurt to spray some insecticidal soap here and there on to the plant. Sometimes it takes care of bugs you can't see.
Mechanical Keyboard Community:
"I need to see these $233 switches!" (Drools)