I have 11 month-olds that weigh about 190 grams so I don't think your guys weigh too little (not totally sure of this).
Are they actually ingesting all of the food? Or are they hiding it?
What are you feeding them?
Do they have an unlimited supply?
How long have you been tracking their weight?
The only serious issue that any of my degus have had associated with weight loss is dental malocclusion, but I would think your guys are too young for that. Perhaps they are burning more calories than they are taking in.
I have also heard that their weight can vary considerably depending on when you weigh them (a full bladder vs an empty bladder, etc.)