I believe the fish have to show up and 'try' the feed before they know the feed is good or bad, so I don't think the age of your feed has to do with the no-shows.

It's certainly odd, but then you did say that you've seen fish, so no reason to believe they're not still in there.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."