Your hair could be dry. Did you deep condition it first before you tried to straighten it? What type of moisturizer are you using? Also, rollersetting first will help smooth the hair.
Sorry, I have no suggestions--my hair does the same thing. I'll just wait for fall or winter to straighten my hair. I just feel no sense in fighting nature.
Same here. I won't even try to straighten my hair in humid/hot weather because there's no point. I just wear wash n' go's or braid-outs until the weather cools down. I wish I could recommend a product that would help but I've never found any.
I press my hair with Aveda Anti-humectant Pomade and Extra Virgin Coconut Oil, and that combo really fights off the humidity. Also when there is high humidity like it is now, I apply a light coating of Sebastian Xtah Raw silk smoothing creme to my edges, nape area, and ends of my hair and it adds an invisible coating to my hair, that slows down hair reversion due to the humidity.