They are notoriously slow growing. Here's some tips that may help. Feed 6-8 times a day with high protein foods. Just small meals. Foods like live black worms, frozen meal blood worms & mysis shrimp. A constant supply of food and not allowing their guts to empty will speed growth.
Massive daily water changes. This will reduce growth inhibiting phermones and chemicals. 100% changes should not be discounted. A minimum of 75% should be the smallest water change.