For those who haven't seen the play "Little Shop of Horrors" it tells the story of an extremely ravenous plant named Audrey II that has an appetite for blood. When he gets hungry he yells out "Feed Me". Lucky for us plants don't really shout this out because it would be pretty noisy. I just had a poll on my blog that just closed asking how often one feeds their annuals. The results stunned me. It shows that I haven't done a good enough job educating our customers on the importance of feeding their plants. Twenty five percent of the people that responded to the poll don't feed their annuals. The plants that we sell do get fed on a regular basis at the grower, and then get fed biweekly once in our care, but this not enough to sustain them through the growing season. The unfed plants won't die, but they will decrease their flower production, and may stop blooming completely. The foliage will lose that lush green color that they had when purchased, and start to yellow. It is even more imperative that you feed your annuals if they are in a container than if they are in the ground. A plant will get some extra nutrients from the soil in the ground, so the malnourished symptoms will take longer to show up. Nutrients in a container will also "leach out" meaning that you will also wash some of the nutrients out of the soil every time you water.
As a horticulturist I could write quite a long article going into the technical aspects of what a plant needs, but I don't want to bore you with too many details. Here's an extremely abbreviated version. Think of a plant as a factory. It takes sunlight, water, and nutrients to produce its product of leaves, flowers, roots, and fruits. If any of the components are missing the finished product is not going to be complete.
It's not too late to feed your annuals now. Use any fertilizer that is labeled for flowering plants. For the quickest results I would use a liquid fertilizer such as Miricle-Gro. (Ok it's not a liquid when you get it, it's a powder but once you mix it with water it's considered a liquid). Follow the directions. Before you feed your plant, you need to make sure the soil is already wet, so it should be watered several hours before you feed it. A quick trick to speed up the time that it takes to feed the plant is to mix it in a watering can, and lightly wet the foliage with the mix, and get the mix on the roots too. Plants can absorb some of the nutrients through the foliage. By using this technique, you will actually start seeing the foliage darken in just a few days, and flower bud production should start to increase in under a week.
Next year start the season by adding a slow release fertilizer such as Osmocote to the pot when you get it home, and then feed with the liquid feed biweekly, and you'll get the best results.