What could cause a toddler not to gain weight when they eat well balanced appetite? 18 month old not gaining weight and in 17th percentile

Need more info: Has your baby always been in the 10-20th percentile for weight? or has her percentiles dropped? if she is consistently in the same percentile range, then I wouldn't worry about her lack of weight gain. However, if she was in the 50-75% for weight when she was 12-15 months old, and now has dropped to 17th percentile, then I am concerned.

Many reasons: It could be that the patient is not eating enough calories, or is expending too many calories or is not absorbing calories into his body. Follow-up with the pediatrician and follow his growth charts to try to figure out the cause.