
There is an abundant number of ways to develop flower buds that still do not open; see which one applies in this case: lack of water or insufficient water; too much water; too much nitrogen from lawn fertilizers (use high phosphorus fertilizers); too little direct sunlight (aim for much more than 6 hours); competition for nutrients from other plants/some trees/weeds/grass/etc.; pruning when the plant is developing flower buds; pest infestation (mealybugs, scale, aphids, etc.);