How much does the food you are giving them weigh? Sounds like you are underfeeding them. You should feed 10% of their body weight for each meal.

Do they ever refuse food? They do seem pretty underweight. Most snakes at that age are 100grams+, although all snakes grow at different rates.

I would take them to the vet just to be safe. A fecal costs about $25 if I remember correctly.