Why won't my canary incubate her eggs?

Hi there, well, I have 2 pairs of canaries that have eggs, the thing is that one of the hens wont lay on them. She has 2 eggs in her nest right now, but no matter what, she wont go near the nest.
My other hen in the other hand, when she had her firs egg, she immediately incubated it. I even touched it to see if it was fertile, and she went right back to it.
my question is why wont the other sit on the nest?

Your canary may not be broody (or physiologically ready) to incubate her eggs. The conditions may not be right for her to hatch chicks, or in her own mentality, she may not think the conditions are right.
sometimes leaving eggs (or fake eggs) in a nest can switch on the broody mood.
some hens never go broody, or might go "half" broody...sit for a week or so..and give up.
what's your weather? The weather, location, age of the hens, and even lighting can play a factor.