You say that the other children are mostly older, up to a year older. I assume that means that your daughter is basically a year ahead of where children normally are, right?
Both from personal experience and from some studies on the matter, I would recommend that in most cases, even very smart or gifted children see benefits from going to school at the normal pace, not ahead. Not every child is the same, but if your daughter is not ahead of the other kids in her grade right now, it sounds like she might be better off waiting and getting into the class when she's of a like age to the other children in it.
The disadvantages to starting early are that even if she keeps up academically, she will be less physically and emotionally developed than her classmates. This means she'll be less able to keep up in sports (because she's younger and smaller), but also less able to keep up in the social world (because she's less emotionally developed). For some children this may not be a particular problem - my four year old is heightwise equivalent to a five year old anyway, so he'd probably keep up fine physically - but from my experience (starting a grade early) it can be difficult being the shortest child in the class, and not understanding why it's hard to make friends.
Research does support advancing very gifted children, particularly if they're in an environment with other very gifted children. Johns Hopkins recommends skipping children with measured IQ of 130+, for example; that seems reasonable to me. It does require the strong support of the parents, the school, and the child, and should only be done if the child feels comfortable in the higher grade.
On the opposite side, many educators believe it's never a good idea because it makes it harder for the child emotionally and physically, in particular hitting middle school and high school early during the difficult time of adolescence.
Overall, I would recommend having a discussion with your child's teachers, and finding out what they think; and perhaps getting some input from others in your life who may have had similar situations and know your daughter well enough to form opinions themselves. My personal feelings are to avoid skipping grades most of the time, due to my own experience; but as each child is different, you should look for professional advice tailored to your daughter's specific needs.