Speak late: Some children are unwilling to clear the way. For example, some children haven't spoken until they are two years old, and some may only speak address forms or overlapping words for a long time, with little progress.
I don't know: the improvement of pronunciation accuracy is slow, which may be related to the slow opening of the mouth in the early stage, because pronunciation is also an action, and it needs constant practice to make the muscle function of the relevant parts reach the standard of accurate pronunciation.
Cognitive impairment in understanding: There may be many reasons, such as poor language comprehension, which adults can't understand or understand to varying degrees.
Children with language disabilities can catch up with the development of normal children through language training.