Learning to speak is very different from learning grammar, it involves specific mouth exercises, pitch and tone and copying how other people speak. I am always very surprised that some people and even some teachers think it happens "naturally". It needs work.
I talk a lot about it in my video blog: www.dailymotion.com/englishwithjoseph you can also try the shadow method which is designed to help. You can find more information on my blog about that too.