Why can't I gain weight? I am very thin and weigh 97. Already have a toddler and tried whey protein ensure eating way more. What can I do?

See your doc. Always been that way? New symptom? Causes can include normal, that's just the way you are; also can be due to poor diet choices, overactive thyroid, parasites, celiac sprue, pancreatic disease, cancer, chronic infection, chronic inflammation, etc. See your doc.