Does powdered hummingbird food go stale?

When I put out our hummingbird feeder in May, I used some powdered food mix left over from last year (and I think some of the packets may be some even older than that). So far I've only seen an occasional hummer stop and check out the feeder, maybe take a sip or two, but then fly on. Now granted it's typically July before any take up long-term residence around our yard, but I am concerned that the food has lost its "zip" in being so old. If so, could I just add some regular sugar to it to revitalize it? Or is it best to just dispose of it?