They should only have 50 grams of veg per day. Overfeeding veg can cause the guts to become unbalanced which results in soft poops.
You need to not feed veg at all until poops normalise. Feed them only hay and the normal limited amount of pellets (one tablespoon per day). 24 hours after poops normalise you can slowly start to add veg back in, starting with herbs is best as they are milder on the tummy. They could be off veg for several days before things go back to normal.
If soft poops persists after being off veg, then see a vet.