If it is hot outside, put the resin in the frige for a bit, dont let it get too cool though, the only draw back to doing this is it sometimes cures a little "cloudy" but does buy more time, also if the hardener you got is old it will do exactly what you explained, the peroxide in the hardener starts to breakdown do to being photo (light) sensitive, (ever wonder why rubbing alchohol comes in a clear container and hydrogen peroxide comes in a brown container?) hardener is cheap.......
If the resin "gelled" and stayed that way for a long time before hardening, I would say definately time for more hardener......The Bondo brand stuff has a chemical in it that changes color when the hardener is added, did it change color (like yellow to green, or brown to blue)?
This is why I use the epoxy crap, you change the hardener for the aplication (almost idiot proof)
Good luck!