How can I make my once curly hair curly again?

So. I had curly hair. It use to be long and I found that the thickness and length would weigh the curls down. From the roots to midway down the hair shaft I would have straight hair and then it would go into ringlets. Everytime I cut my hair (above shoulder length) my hair never curls. I use curling products and a diffuser and even a curling iron but it just doesn't curl. It also doesn't stay straight but goes . I mentioned this to my stylist and when she styled my hair (product, then a diffuser) it curled. But me? not so much. Any help?