Great Tit eggs not hatching?

Hello,

I have a Great Tit nesting in my camera bird box and she began to lay eggs on the 28th April. After a few weeks it looked like none of the eggs had hatched even tho she had been on the nest constantly. She then left for a day or two and I was going to empty the nest but she came back and began sitting on the nest again. She then laid another 5 eggs and has been sitting on all of eggs for about 2 weeks. 

I feel very bad for her as it looks like this set will not hatch either. Will she continue to keep going? Is there a reason why the eggs are not hatching?