You may also want to see a ENT and check her hearing. If you live in the US your child may qualify for the Early Intervention Program or ELP and receive language and speech therapy in the public school system at no cost to you. You may want to contact you school district for more info and or get a referral from your DR requesting the school district test your child.
while it's possible that it's a normal part of her development, I would be concerned. She most likely needs speech/language therapy. I am surprised that her pediatrician isn't concerned. in the mean time, if she is responding well to flash cards, i would create activities that foster language development. help her learn to put sentences together by modeling and have her repeat you (as you are). Since she does well with flashcards, you may want to look at http://www.speakingofspeech.com/Materials_Exchange.html. That site has many different activities for many levels. some of them are picture prompts that encourage the child to speak in sentences. You point to each picture and have her repeat it. but most importantly, speak to her pediatrician. you might also be able to contact your school district and inquire about early intervention.