Not Enough Sunlight For Cosmos to Bloom
Cosmos are native of Mexico and South Africa. They like the sun.
If your cosmos have been planted in a shady area of your garden then they tend to grow leggy in search of more light and with few flowers.
The amount of sunlight is one of the most influential factors for how well your cosmos flower.
Nutrient Rich Soil and too much nitrogen rich feed prevents flowering
As previously mentioned, cosmos are native to Mexico where they grow wild and flower in great number in sandy soil that is low in nutrients and very well draining.
If your garden soil is rich in nutrients and has had amendments such as compost, or manure added to it recently then it is likely that the soil is too rich for cosmos to flower.
Nitrogen feed leads to cosmos putting on green growth at the expense of flowering. Essentially you are coddling your plants. A period of neglect is no bad thing.
Watering Cosmos Too Frequently Causes Fewer Flowers
Cosmos prefer a soil that is well draining and dries out somewhat after watering or rain as this is the conditions to which it is adapted. They thrive in full sun. Therefore if cosmos are to flower, you should refrain from over watering the soil.
Cosmos are very hardy when it comes to tolerating drought like conditions. If the soil is kept consistently moist then this can promote healthy foliage with few flowers.
Cosmos thrive in conditions that are fairly harsh as it is adapted to sandy soils in arid climates.
To promote flowers for cosmos that may be not flowering due to over watering, you should scale back the watering so that the soil has a chance to dry out between bouts of water.
Timing of Planting Can Affect Blooms
The optimal time for sowing seeds of cosmos is around March or April or after the threat of frost.
If you sow seeds after the optimal window of March and April then this can delay flowering for some weeks, however with patience and the right conditions the cosmos can still bloom.
Sowing in the middle of Summer can often be detrimental as the cosmos has less time to germinate, and has to contend with particularly high temperatures as it grows and it the flowering period may be late enough in the year that there could be a threat of frost which damages the flowers.
Planting cosmos too early can also risk frost damage from a late frost in the very early Spring which can prevent flowers.
To ensure your cosmos are safe from the risk of frost in the early Spring, it is a good idea to sow the seeds indoors in a greenhouse to protect them whilst they are still tender and give the seedlings time to grow in the early Spring so they can flower as early as possible and for a long time in the Summer.
Short Day Flowering- Cosmos Only Flowers When Days are Shorter then Nights
Cosmos can often flower best in late Summer or Autumn when as they tend to flower best when the day length is less the 12 hours (short day length plants).
Whilst full sun is beneficial for flowering if cosmos are in bright light for longer then 12 hours then this can impact flowering as the require a period of darkness.
This is due to the cosmos being adapted to sense seasonal change by the pattern of changes in day length so that the plant knows when to flower.
In the northern hemisphere the longest day is on the 21st of June, so cosmos start to develop flowers after this date as the days grow shorter.
If you have a source of light at night that may light up around your cosmos (and other short day plants) then this can reduce flowering even if it is just for a few minutes.
Think of artificial sources of light such as street light or security light that are bright after dark as cosmos require 12 hours of consistent darkness to flower.
In Northerly latitudes the day length tends to be longer which causes the cosmos to flower later in the Summer and perhaps into the Fall.
If you can successfully block out light sources (with horticultural fleece or something similar) so that the day length and hours of light is less then 12 hours then this can help to promote flowers at any time during the Spring or Summer or you can wait until late Summer or Fall for flowering.
Clay Soil Can Prevent Flowering
Clay soil (particularly heavy clay) is not well suited for growing cosmos as it is rich in nutrients (which promotes foliage at the expense of flowers) and does not drain very well and cosmos prefers well draining sandy soil.
If the soil is consistently damp then cosmos does not necessarily flower.