How do you heat a hard candy mixture to 300 degrees?

I tried to make hard candy the other night and the highest I could get the mixture to reach on the stove top was around 240 degrees. I had the lid on and used a candy thermometer and everything, but it just wouldn't seem to get much hotter. Consequently, the hard candy turned out more like taffy. Does it need to be left on the stove for hours to reach 300? I suggested putting it in the oven to my husband, and he says an oven's heat is a different kid of heat (exothermic?). I'm no master in the kitchen, so any suggestions you have are appreciated?