
Typically, unless they are still relatively young, the plants should bloom shortly (within a few weeks) of reaching their mature height. It could be that there is a nutrient imbalance in the soil - likely too much nitrogen, which results in plenty of foliage growth, and too little phosphorus, which is necessary for blooming. You can remedy this by using a high phosphorus fertilizer (bloom booster) or by adding bone meal to the soil around your plants. Here is more information: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm