The teacher should give you clear examples of problems your child is having and information about normal development. Severe separation anxiety, difficulty learning basic kindergarten skills or trouble relating to other children could be reasons that another year of kindergarten might help your child.
If the teacher is concerned your child has a learning disability, holding her back might not help by itself. In this case, medication, occupational therapy and help from a developmental psychologist can help get your child ready for first grade; if developing the right treatment plan will take several months, holding your child back could give her the time she needs.
Your child's teacher should be well-informed about developmental issues in kindergarten, so rely on her as an expert, rather than assuming she's wrong. Ask about various options for helping your child succeed. The teacher might recommend an evaluation, additional time on learning activities at home or help with mastering social skills. She might also suggest alternatives to holding your child back, such as tutoring, summer school or lifestyle changes.
If you're concerned that holding your child back will stigmatize him, lower his self-esteem or make it difficult for him to make friends, you can express these concerns. Ask the teacher about what can be done to mitigate the effects of holding your child back and inquire about the specific benefits and drawbacks of delaying first grade. If the teacher can't offer a coherent plan for ensuring your child does not suffer due to the decision to hold her back, you might need additional guidance.
Before you make the final decision to hold your child back, consider getting a second opinion. Talk to the principal or consult with another teacher who has worked with your child. If you're unsure of what's going on with your child or unclear on what you can do to help him, talk to your pediatrician or get a referral to a child psychologist, who can help you develop a comprehensive plan.