There could be a multitude of reasons for this behavior. I presume his hearing is normal as he speaks fluently. I would recommend to see a pediatrician who would be in a better position to counsel you on this aspect.
I understand, your daughter's hearing is normal and she has problems in expressing her thoughts clearly. This seems a cognitive issue. I would suggest consulting a pediatrician and speech language pathologist.
I hope this information is helpful and you have access to these specialists in your area.