My niece is 18 months, and she struggles from anywhere between 1-3 hours to poop. She screams and…

My niece is 18 months, and she struggles from anywhere between 1-3 hours to poop. She screams and cries and when she finally gets it out, there's hardly anything there. My sister-in-law asked her doctor, and the doctor prescribed a stool softener, but it hasn't helped, even a little bit. Has anyone else been through this? Any advice?