Well, without specifics I can't say for sure, but probably the biggest issue is that some of our support staff are scared of taking callbacks, because callbacks take longer to handle and often involve complex issues. We try to remind them that callbacks are important, but sometimes they forget!
(Callbacks for angry customers are particularly unattractive, for obvious reasons. Very few of our support staff enjoy having their ears shouted off.)
You're right that it's an area for improvement, though. We're working on it.