Answer: Without details on how the plant is cared for or where it's growing it's difficult to say. However, sun exposure and nutrient levels in the soil are typical problems for lack of flowering. Lantana needs full sun to bloom, but isn't terribly fussy about nutrients if it's growing in the ground. Diascia needs partial shade in desert climates, protection from afternoon sun.
If either plant is growing in a container, are you fertilizing regularly? Frequent watering will leach away nutrients, so they need to be reapplied. Here?s some basic info on fertilizer and nutrients that plants require. The 3 numbers on a fertilizer bag refer to the percentage of N (nitrogen), P (phosphorus), and K (potassium) in the bag. There are different formulations for different purposes. In general terms, nitrogen produces lush green growth, phosphorus helps strengthen stems and produce flowers, and potassium keeps the root system healthy. If you're applying fertilizer to fruiting (e.g., tomatoes) or flowering plants, you're not as interested in the plant developing leaves as you are in it flowers and fruit, so you'd use a formulation lower in nitrogen and higher in phosphorus, such as Miracle-Gro's Plant Food at 15-30-15. Bone meal is an organic source of phosphorus.
How often to water depends on many factors including plant type and size, soil type, weather, and even your particular microclimate. Lantana in the ground is low water use groundcover. Water when the top inch or two of soil dries out. Water should soak to a depth of 12 inches. Annuals and perennials in the ground generally need more frequent watering, especially in summer. In a pot, they may need daily water. Hope this info helps!
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