• Knock Out roses are a family of shrub roses developed by the Conard-Pyle Company of Pennsylvania. According to their website, Knock Out roses are the most widely sold roses in North America. These popular shrub-type roses are available in red, pink and yellow, in both single and double forms. Most Knock Out roses will grow to 3 to 4 feet high and wide if left unpruned. If your Knock-Out roses aren't blooming, there could be several causes.
  
  

Light Requirements

• Knock Out roses need at least six hours of direct sunlight daily to bloom well. Move them to a sunnier spot if they're receiving less than that.
  
  

Flowering Cycles

• Knock Out roses typically bloom in cycles, with an abundant first flush of flowers followed by a lull in flowering for five or six weeks, according to Conard-Pyle's online information. There may be occasional flowers produced during the rest periods. This cycle of a heavy flush of flowers followed by a rest period continues throughout the summer until heavy frost.
  
  

Improper Fertilization

• The American Rose Society recommends using a fertilizer specially balanced for roses with a 6-12-6 ratio. Those numbers stand for the percentages of nitrogen, phosphorus and potassium found in the plant food. Phosphorus encourages root and bloom growth, so the important thing to remember is that rose food should have twice the amount of phosphorus (the middle number) as it does nitrogen and potassium. Using a fertilizer high in nitrogen, such as one intended for lawns, will result in vigorous leaf growth at the expense of flowers.
  
  

Soil Too Acid or Alkaline

• Test your garden to determine its pH, which indicates the relative acidity or alkalinity of the soil. You can purchase a pH meter at hardware stores or garden centers, and many cooperative extension services offer inexpensive pH testing. Knock Out roses prefer a neutral soil, with a pH of around 7.0. A pH too far above or below that value makes nutrients unavailable to plant roots. Consult with your local extension service or garden center for advice about additives if you need to adjust the pH of your garden soil.