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, growing out mechanical and chemical damage = breakage and very high porosity. Past armpit length heading for waist.
Treatments: Komaza protein, Komaza Matani, coconut oil with sugar syrup. Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss ultimate volume
Styler: Umberto Giannini jelly, Boots curl creme, Rockin RInglets
Experimenting with: Mega-Tek, Curl Harmony reviver, Elvive Full Restore 5.