My hair wont straighten :(?

Ok so my hair semi straightens but its a poofy straight. Is it a root problem? Im black but I have relaxed hair n even brfore I ws relaxedi actually had wavy hair. I use th instyler but also have a RED ceramic straightener I use tresemme heat protectant. I heard chi silk infusion works well but my mom wont let me buy bc it wont work on black hair despite th many videos ive seen -_-. Whts th problem n how cn I get my hair straighter