Why didn't my jello set? I found a box of lime jello that I have had for a while (at least 6 months). I made it last night, following the directions (boiling water, etc), and left it in the fridge til today. I went to eat it just now, 24 hours later, and it is only set in parts - there is still lots of liquid, and it isn't as jelled as I would expect - it's chunky, not a solid wiggling mass of jello goodness.

Steps I took: 

Boiled water (a bit more than a cup) in a pot on my stove. Measured one cup of the boiling water, poured it into a pyrex bowl, added the packet of lime jello powder. Mixed it until it was dissolved, then added one measured cup of cold water.  Stirred it a bit, then put the lid on the bowl and put the bowl in my fridge. Didn't add any fruit or anything. 

What could have gone wrong?
Covering jello while it sets is not usually a problem. At my last job the pantry person covered the jello every time it was made while setting. This was due to health code regulations. Having the jello for that long usually isn't an issue either unless there was some way moisture got to the box and may have compromised the gelatin or even caused mold growth. Was the box expired?