Are you sure they're not just rootbound or maybe have a nutrient lockup due to pH imbalance? Try flushing them thoroughly, a lot of times if you haven't flushed them in awhile, thats what they need, and will take off afterwards. As long as they look nice and green and healthy, they might just need more space for their roots. What size pots you got them in and how tall are they? A general rule is 1 gallon of pot size for every foot of height.