If your hair is not growing in curly then it's quite possible your natural curl pattern has changed since you began straightening it. You can try to boost curl by experimenting with hydrolysed protein treatments, or coconut oil and honey soaks, or products containing magnesium suphate (epsom salts) like Jessicurl Rockin Ringlets. BUT the protein and mag sulph can both be drying so do not overuse them.

If it is mainly the lengths that are not maintaining curl you probably have more damaged hair remaining than you think. Even if your hair has grown ten inches in a year you must still have some hair that has been flat ironed in the past.
2a-2c, medium texture, porous/ colour treated. Three years CG. Past bra strap length heading for waist.

CO-wash: Inecto coconut/ Elvive Volume Collagen
Treatments: Komaza Care Matani, coconut/ sweet almond/ fractionated coconut oils, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss Ultimate Volume, various Elvive
Styler: Umberto Giannini jelly, Au Naturale styling gelee
Flour sack towel, pixie diffuse or air dry.
Experimenting with: benign neglect