It sounds under processed. Generally the back of the head takes more time to process than the hairline and the crown. I'd go back and ask if they'll correct it. They probably will, free of charge.
And BTW, yes chemical treatments are bad for your hair. They're addictive too, as relaxers permanently straighten your hair, and with new growth you're going to want to touch it up or it'll look a bit off. Be very careful with these relaxers.
.... Wait, Japanese Straightening? So you've had a relaxer on your hair before? Girl you're playing with fire...