Certified GKH Gardening Expert
What zone are you located in? This could have something to do with temperature. These need a cold period to recharge its ability to bloom.
Alternatively, there may be too much nitrogen, or not enough phosphorus and potassium to support healthy blooming. You can correct this by adding a high phosphorus and potassium fertilizer. This will offset the nitrogen.
Adding dolomitic lime can also help raise the pH if it is too low. This can make nitrogen more available while locking out phosphorus and potassium. Bringing this back up to neutral will help set the balance back.