Help! How do help my dog gain weight

My puppers is about 1 1/2 years old and has seemingly been losing weight she’s a free fed dog and her bowl is always full, she has been cleaning up my kids food they drop (that’s dog safe at least) but now it seems she is waiting to eat until she realizes shes not getting human food, I’ve started putting toppers on her food to encourage her to eat but it only seems like she’s eating what the topper is on any tips for lower income