Ask Mr. Smarty Plants

The question to ask yourself is;  What has changed from when the plants flowered to the present?
Are the plants getting more or less water now, or are they getting more fertilizer now.

There are two plants in our Native Plant Database with the common name blue mist flower;  Chromolaena odorata (Jack in the bush) and Conoclinium coelestinum (Blue mistflower).

I'm not sure which plant we are talking about here, but there are plants whose flowering is inhibited by an imbalance in the the nutrients that are available in the soil, particularly the ratio of nitrogen to phosphorus (N/P). Too much nitrogen may result in poor flowering. If the plants have been getting regular lawn fertilizer which often has a high N/P ratio, this could account for the lack of flowering.

Lets look at fertilizer a bit more closely.
The most common elements that are found in fertilizers are nitrogen (N), phosphorus (P), and potassium (K), and fertilizers containing these three elements are termed complete fertilizers. Fertilizers differ in the relative amount of these elements that they contain, and this information is found on the label of the fertilizer container (bag, bottle, etc.) It is represented by three numbers eg 1-1-1, or 5-10-5, or 3-1-2, which are the percentages of the elements; N, P, and K in that order. 
This link to  Central Texas Gardening has some good tips about proper use of fertilizers .