Just to clarify, are we talking about [bondo] fiberglass resin, or bondo body fill?
If it's body fill and blue hardener we're talking about...Phil is right about the mix, and isn't what I thought you had.
If it is the resin, and you got the 1st one to harden, you've got the liquid hardener mix and technique down. I think the other can was opened, or exposed to air. Was is more gel-like out of the can? I thought my can was bad, because it was half used, and sat for months.
You may have to scrape it off...try to sit it out in the sun to warm it up and hopefully harden. Maybe someone with more experience knows. I never had any real trouble with fiberglass.