Speech is a form of expression that is not just a function or mouth and lips or vocal apparatus.
Apart from above following are more common causes
A child who cannot hear.. deafness
A child who can hear but cannot listen, attention deficit
A child who gives attention but cannot understand, cognition deficit
A child who can understand but cannot express, apraxia/aphasia
Apraxia of speech: non verbal Communication deficit
Aphasia of speech: verbal communication deficit
Often if the child has behaviour issues in addition, there is also likely hood or poor understanding and possibility of autism.
Speech delay lasting more than three months is itself a form of disability
Other speech issues are speech paucity, echolalia, only need based speech,poor vocabulary issues related to sentence formation, issues of syntax and grammer and speech with poor composition and poor comprehension