What Causes Impaired Speech In A Child?

Hi, I have a daughter who is 2 years and 3months. She has never talked, she used to say masks, daadada but now she stopped. She just humme. She seem not to understand or recognise her sisters and us because when U come in she doesn t seem to care. She has been seen by pediatric and she said she s ok and sent her for ears/eyes to be checked. These were checked and all come back clear. What could be affecting my daughter s apech? Please help because I am so worried and crying all the time.