Slow weight gain

My 4 month old only gains 100g (around 3.5oz) per week since birth. I dont know what else to do. She was initially BF then formula topped up. She now is mixed fed to keep up with her requirements. At times Ive tried to force her to drink but when shes done shes done. She poo's every 2nd day and has many wet nappies a day (around 6) Shes otherwise a happy baby but just gaining weight slowly. What else can I do? Should I have her checked out medically even though she has no symptoms?