Head-butting & Refusal to Talk

I have an 18-month old toddler girl who is refusing to talk; she will say Momma & Daddy, "this" & "that", but nothing else. I'm a stay-at-home mom and constantly talk to her and ask her questions, read +6 books per day outloud to her, and sing all kinds of songs to her to help promote mimicry. She doesn't have any hearing problems, and is interactive with me, so I don't think it's learning difficulties. She just shakes her head and waves her hands no whenever we try to get her to parrot what we say.
My toddler also has decided that the best way to express her frustration is to Head-butt my husband or myself, but then she cries because it doesn't feel good. (She does it really hard.) It's normally an issue when she is tired, mostly. 
Have any of you had similar experiences with your toddlers? Do you have any advice on how to curb the head-butting? Or promote more verbalized speech? 
Thank you, in advance, for any advice!