Speech delay. Nd what is the problem?

My 3 year old daughter hardly speaks 7-8 words. We are constantly trying to make her talk. But she mostly explains things using signs nd by touching things doing the acts(like pretending to kick a ball without a football or hold a pen and bite it when she wants a chocolate) nowadays she gets irritated when we don't under her signs or she finds it difficult to explain. She'll put her finger on her tounge nd neck when we ask her to repeat words after we say.. as if she's saying that she Cnt say it. We have seen her trying, she tries a lot but can't speak. We hv seen many doctors they say that she's totally fine but we haven't done any kind of test yet. What should we do ?? What are the tests that could help us to find where nd what is the problem??