Involved parents are so important when it comes to helping their children learn. Can you be a little more specific in what your daughter is having trouble learning? Does she know her letter and their sounds? Is she able to rhyme? Can she retell a story or event? Can she recognize and write her name? Can she count and recognize letters? Can she sort objects and tell you why they are grouped that way? Does she recognize different colors? Does she have any trouble seeing or hearing? Can she follow one and two step directions?
As you can see there is a lot to be learned in kindergarten. Some children pick them up faster than others. And the reasons could be endless as to why. It could because of their birth date, sex, birth order, lack parent involvement (which doesn't sound like you), preschool teachers, etc..
In the meantime, I suggest reading to her daily. One of the most effective ways to help children is to read to them daily. There are some many wonderful picture books that can help to develop all types of literary skills, increase vocabulary and build basic knowledge. If you haven't read Reading Magic (for parents) by Mex Fox, I suggest starting there. She give parents great information on how to make the most of story time.
So to provide you with more specific help, please give us more information. You are doing the right thing!
I would look into other ways to help her learn. You're doing a great job spending time with her and helping her with her homework every night. But if that's not enough you might need to broaden the areas you work on.
Perhaps she needs additional worksheets to help understand concepts. Or maybe it's after school tutoring presented in a different way than your nightly homework help. Or maybe she doesn't learn well at night and studying more on the weekends during the day would work better?
Try different things with her and hopefully you'll discover the best learning method for her.
First I would ask how old the child is. If she is much younger than the rest of the class, it could be that she is just not ready for Kindergarten. Kindergarten expectations are much different than they were even 15 years ago, a child is expected to learn letters, do simple math and phonics.
If that is not the case, then I suggest you speak to her teacher. If the teacher indicates your daughter is behind, or needs extra help, then you will know there may be a problem to address. Perhaps she needs a different type of phonics or whole word learning, not everyone learns the same way. If she has homework every night, likely she is not getting things done in class? If it continues to be a problem, she needs to be screened for speech or learning issues. Your school district will do this for free. Talk to your special education department and get the ball rolling.