Not Perfectly Straightened Hair
A: The answer depends on the type of straightening you've had done to your hair.
If the hair was straightened with a thio-relaxer/straightener, you can usually use another thio-based product over it, provided the hair is in good condition, and has not been damaged by the previous service too severely. Ogilvie Home Straightener and EasyStraight are two thio-based home straightening kits that are available on the market today.
You should under no circumstances consider using a thio-straightener over a hydroxide product (or vice versa). The result will be simply that you'll leave your hair behind in the sink.
My best advice for you is to go back to the salon where you had your hair straightened and explain your dissatisfaction and ask the stylist what can be done. Many times the salon will be willing to help make you happy with the service. However, if the salon or stylist insists on charging you for another service, I would at the very least demand a discount.
©Hairfinder.com
See also:
Permanent hair straightening
How soon after thermal straightening can I wash/wet/style my hair?
Can I use a straightening and curling iron after a bio ionic straightening treatment?