Bright children can have just as much difficulty completing homework as any other child, just for different reasons. You should talk to her and find out why; and it may take some significant amount of time, and conversations that have to be positive in focus - not disciplinary, but friendly.
Gifted students often refuse to do homework because it is boring, uninteresting, unchallenging. When you understand all of the topics as soon as your teacher gives you the lesson, you don't need to waste hours writing math problems! Except that schools don't work that way - lots of kids do need to do the homework to learn, so you do too.
Another common problem is students may have peer pressure to not look 'too bright', and so don't do their homework to try and fit in. Chapter 11 of Emotional Intensity in Gifted Students2 discusses this in part ("Conformism and Peer Pressure"). The suggestion from that chapter is to help the student understand the long-term consequences of these actions, and to help them realize their relationships don't need to be harmed by their academic performance and vice versa.
Gifted students also can be perfectionists, and struggle completing work if they don't think they can complete it perfectly. Chapter 12 of Emotional Intensity in Gifted Students2 has some great examples of conversation around this topic, in particular a great sample conversation with the child.
In particular, if this is the problem you probably will already realize your child is somewhat of a perfectionist if you pay attention to her signals. Does she dislike showing her drawings to you when they're not completely finished (and even then maybe not)? Does she erase a lot? Does she get stuck on a single problem and not want to leave it?
Perfectionists often struggle with homework because they don't want to turn it in unless they'll get a 100% on it, and they will focus for a long time on a problem that is difficult and have a hard time going past it if they get stuck - while also sometimes having a hard time asking for help.
If this is the case, then my suggestion (other than to read Emotional Intensity in Gifted Students, linked below) is to talk over her issues with her, and try to give her some techniques for dealing with this stress/anxiety. You may also want to involve a psychologist, either from your school or separate, to help her learn coping mechanisms.
1 The Davidson Institute has many great resources for parenting gifted children, in addition to their book (Genius Denied) which gives a lot of examples of highly gifted children who struggled in their school system. Also, their resources page has many other helpful links.
2 Emotional Intensity in Gifted Students* (Christine Fonseca, Prufrock Press, 2011) - this discusses a lot of challenges of gifted students from a more psychological standpoint.